implement chat messaging with event proposals

- Add functional chat with server communication and test responses
- Add ProposedEventCard component for confirm/reject actions
- Move Constants (Day, Month) from client to shared package
- Add dateHelpers utility for weekday calculations
- Extend Themes.tsx with button and text colors
- Update CLAUDE.md with current implementation status
- Add *.tsbuildinfo to .gitignore

packages/shared/src/utils/dateHelpers.ts

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+import { Day, DAY_INDEX } from '../models/Constants';
+
+/**
+ * Get a date for a specific weekday relative to today.
+ * @param day - The target day (e.g., "Friday")
+ * @param offset - 1 = next occurrence, 2 = the one after, -1 = last occurrence, etc.
+ * @param hour - Hour of day (0-23)
+ * @param minute - Minute (0-59)
+ */
+export function getDay(day: Day, offset: number, hour: number, minute: number): Date {
+  const today = new Date();
+  const currentDay = today.getDay();
+  const targetDay = DAY_INDEX[day];
+
+  let daysUntil = targetDay - currentDay;
+
+  if (offset > 0) {
+    // Future: if target is today or past, move to next week
+    if (daysUntil <= 0) daysUntil += 7;
+    daysUntil += (offset - 1) * 7;
+  } else if (offset < 0) {
+    // Past: if target is today or future, move to last week
+    if (daysUntil >= 0) daysUntil -= 7;
+    daysUntil += (offset + 1) * 7;
+  }
+
+  const result = new Date(today);
+  result.setDate(today.getDate() + daysUntil);
+  result.setHours(hour, minute, 0, 0);
+  return result;
+}

packages/shared/src/utils/index.ts

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+export * from './dateHelpers';