refactor: improve AI event handling and conflict display in chat

- AI fetches events on-demand via callbacks for better efficiency
- Add conflict detection with warning display when proposing overlapping events
- Improve event search and display in chat interface
- Load full chat history for display while limiting AI context

CLAUDE.md

@@ -266,10 +266,10 @@ src/
 │   ├── index.ts              # Re-exports GPTAdapter
 │   └── utils/                # Shared AI utilities (provider-agnostic)
 │       ├── index.ts          # Re-exports
-│       ├── eventFormatter.ts # formatExistingEvents() for system prompt
+│       ├── eventFormatter.ts # Re-exports formatDate/Time/DateTime from shared
 │       ├── systemPrompt.ts   # buildSystemPrompt() - German calendar assistant prompt
 │       ├── toolDefinitions.ts # TOOL_DEFINITIONS - provider-agnostic tool specs
-│       └── toolExecutor.ts   # executeToolCall() - handles getDay, proposeCreate/Update/Delete, searchEvents
+│       └── toolExecutor.ts   # executeToolCall() - handles getDay, proposeCreate/Update/Delete, searchEvents, getEventsInRange
 ├── utils/
 │   ├── jwt.ts                # signToken(), verifyToken() - NOT USED YET (no JWT)
 │   ├── password.ts           # hash(), compare() using bcrypt
@@ -325,10 +325,12 @@ src/
 - `CalendarEvent`: id, userId, title, description?, startTime, endTime, note?, isRecurring?, recurrenceRule?, exceptionDates?
 - `ExpandedEvent`: Extends CalendarEvent with occurrenceStart, occurrenceEnd (for recurring event instances)
 - `ChatMessage`: id, conversationId, sender ('user' | 'assistant'), content, proposedChanges?
-- `ProposedEventChange`: id, action ('create' | 'update' | 'delete'), eventId?, event?, updates?, respondedAction?, deleteMode?, occurrenceDate?
+- `ProposedEventChange`: id, action ('create' | 'update' | 'delete'), eventId?, event?, updates?, respondedAction?, deleteMode?, occurrenceDate?, conflictingEvents?
   - Each proposal has unique `id` (e.g., "proposal-0") for individual confirm/reject
   - `respondedAction` tracks user response per proposal (not per message)
   - `deleteMode` ('single' | 'future' | 'all') and `occurrenceDate` for recurring event deletion
+  - `conflictingEvents` contains events that overlap with the proposed time (for conflict warnings)
+- `ConflictingEvent`: title, startTime, endTime - simplified event info for conflict display
 - `RecurringDeleteMode`: 'single' | 'future' | 'all' - delete modes for recurring events
 - `DeleteRecurringEventDTO`: mode, occurrenceDate? - DTO for recurring event deletion
 - `Conversation`: id, userId, createdAt?, updatedAt? (messages loaded separately via lazy loading)
@@ -342,6 +344,39 @@ src/
 - `Day`: "Monday" | "Tuesday" | ... | "Sunday"
 - `Month`: "January" | "February" | ... | "December"
 
+### AI Context Architecture
+
+The AI assistant fetches calendar data on-demand rather than receiving pre-loaded events. This reduces token usage significantly.
+
+**AIContext Interface:**
+```typescript
+interface AIContext {
+  userId: string;
+  conversationHistory: ChatMessage[];  // Last 20 messages for context
+  currentDate: Date;
+  // Callbacks for on-demand data fetching:
+  fetchEventsInRange: (start: Date, end: Date) => Promise<ExpandedEvent[]>;
+  searchEvents: (query: string) => Promise<CalendarEvent[]>;
+  fetchEventById: (eventId: string) => Promise<CalendarEvent | null>;
+}
+```
+
+**Available AI Tools:**
+- `getDay` - Calculate relative dates (e.g., "next Friday")
+- `getCurrentDateTime` - Get current timestamp
+- `proposeCreateEvent` - Propose new event (includes automatic conflict detection)
+- `proposeUpdateEvent` - Propose event modification
+- `proposeDeleteEvent` - Propose event deletion (supports recurring delete modes)
+- `searchEvents` - Search events by title (returns IDs for update/delete)
+- `getEventsInRange` - Load events for a date range (for "what's today?" queries)
+
+**Conflict Detection:**
+When creating events, `toolExecutor` automatically:
+1. Fetches events for the target day via `fetchEventsInRange`
+2. Checks for time overlaps using `occurrenceStart/occurrenceEnd` (important for recurring events)
+3. Returns `conflictingEvents` array in the proposal for UI display
+4. Adds ⚠️ warning to tool result so AI can inform user
+
 ### Database Abstraction
 
 The repository pattern allows swapping databases:
@@ -402,7 +437,6 @@ The decorator uses a Proxy to intercept method calls lazily, preserves sync/asyn
 **Log Summarization:**
 The `@Logged` decorator automatically summarizes large arguments to keep logs readable:
 - `conversationHistory` → `"[5 messages]"`
-- `existingEvents` → `"[3 events]"`
 - `proposedChanges` → logged in full (for debugging AI issues)
 - Long strings (>100 chars) → truncated
 - Arrays → `"[Array(n)]"`
@@ -474,9 +508,11 @@ NODE_ENV=development        # development = pretty logs, production = JSON
   - `MongoChatRepository`: Full CRUD implemented (getConversationsByUser, createConversation, getMessages with cursor pagination, createMessage, updateMessage, updateProposalResponse, updateProposalEvent)
   - `ChatRepository` interface: updateMessage(), updateProposalResponse(), updateProposalEvent() for per-proposal tracking
   - `GPTAdapter`: Full implementation with OpenAI GPT (gpt-4o-mini model), function calling for calendar operations, collects multiple proposals per response
-  - `ai/utils/`: Provider-agnostic shared utilities (systemPrompt, toolDefinitions, toolExecutor, eventFormatter)
-  - `ai/utils/systemPrompt`: Includes RRULE documentation - AI knows to create separate events when times differ by day, warns AI not to put RRULE in description field
-  - `ai/utils/toolDefinitions`: proposeUpdateEvent supports `isRecurring` and `recurrenceRule` parameters for adding UNTIL or modifying recurrence
+  - `ai/utils/`: Provider-agnostic shared utilities (systemPrompt, toolDefinitions, toolExecutor)
+  - `ai/utils/systemPrompt`: AI fetches events on-demand (no pre-loaded context), includes RRULE documentation, warns AI not to put RRULE in description field, instructs AI not to show event IDs to users
+  - `ai/utils/toolDefinitions`: proposeUpdateEvent supports `isRecurring` and `recurrenceRule` parameters, getEventsInRange tool for on-demand event loading
+  - `ai/utils/toolExecutor`: Async execution, conflict detection uses `occurrenceStart/occurrenceEnd` for recurring events, returns `conflictingEvents` in proposals
+  - `MongoEventRepository`: Includes `searchByTitle()` for case-insensitive title search
   - `utils/recurrenceExpander`: Handles RRULE parsing, strips `RRULE:` prefix if present (AI may include it), filters out exceptionDates
   - `logging/`: Structured logging with pino, pino-http middleware, @Logged decorator
   - All repositories and GPTAdapter decorated with @Logged for automatic method logging
@@ -538,7 +574,7 @@ NODE_ENV=development        # development = pretty logs, production = JSON
 - `ModalBase`: Reusable modal wrapper with backdrop (absolute-positioned behind card), uses CardBase internally - provides click-outside-to-close, Android back button support, and proper scrolling on Android
 - `EventCardBase`: Event card with date/time/recurring icons - uses CardBase for structure
 - `EventCard`: Uses EventCardBase + edit/delete buttons (TouchableOpacity with delayPressIn for scroll-friendly touch handling)
-- `ProposedEventCard`: Uses EventCardBase + confirm/reject/edit buttons for chat proposals, displays green highlighted text for new changes ("Neue Ausnahme: [date]" for single delete, "Neues Ende: [date]" for UNTIL updates). Edit button allows modifying proposals before confirming.
+- `ProposedEventCard`: Uses EventCardBase + confirm/reject/edit buttons for chat proposals, displays green highlighted text for new changes ("Neue Ausnahme: [date]" for single delete, "Neues Ende: [date]" for UNTIL updates), shows yellow conflict warnings when proposed time overlaps with existing events. Edit button allows modifying proposals before confirming.
 - `DeleteEventModal`: Delete confirmation modal using ModalBase - shows three options for recurring events (single/future/all), simple confirm for non-recurring
 - `EventOverlay` (in calendar.tsx): Day events overlay using ModalBase - shows EventCards for selected day
 - `Themes.tsx`: Theme definitions with THEMES object (defaultLight, defaultDark) including all color tokens (textPrimary, borderPrimary, eventIndicator, secondaryBg, shadowColor, etc.)