feat: message bookmarks — star important messages and jump back to them

- Add bookmark button on message hover (amber star icon)
- Bookmarked messages show a small star indicator in the timestamp row
- Floating bookmarks panel to list and jump to bookmarked messages
- Bookmarks persisted in localStorage per session
- i18n support (EN/FR)

src/hooks/useBookmarks.ts

@@ -0,0 +1,68 @@
+import { useState, useCallback } from 'react';
+
+const STORAGE_KEY = 'pinchchat-bookmarks';
+
+export interface Bookmark {
+  messageId: string;
+  sessionKey: string;
+  preview: string;
+  timestamp: number;
+  bookmarkedAt: number;
+}
+
+function loadBookmarks(): Bookmark[] {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY);
+    if (raw) return JSON.parse(raw) as Bookmark[];
+  } catch { /* noop */ }
+  return [];
+}
+
+function saveBookmarks(bookmarks: Bookmark[]) {
+  try {
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(bookmarks));
+  } catch { /* noop */ }
+}
+
+export function useBookmarks() {
+  const [bookmarks, setBookmarks] = useState<Bookmark[]>(loadBookmarks);
+
+  const toggle = useCallback((messageId: string, sessionKey: string, preview: string, timestamp: number) => {
+    setBookmarks(prev => {
+      const exists = prev.some(b => b.messageId === messageId);
+      const next = exists
+        ? prev.filter(b => b.messageId !== messageId)
+        : [...prev, { messageId, sessionKey, preview: preview.slice(0, 120), timestamp, bookmarkedAt: Date.now() }];
+      saveBookmarks(next);
+      return next;
+    });
+  }, []);
+
+  const isBookmarked = useCallback((messageId: string) => {
+    return bookmarks.some(b => b.messageId === messageId);
+  }, [bookmarks]);
+
+  const getForSession = useCallback((sessionKey: string) => {
+    return bookmarks
+      .filter(b => b.sessionKey === sessionKey)
+      .sort((a, b) => a.timestamp - b.timestamp);
+  }, [bookmarks]);
+
+  const remove = useCallback((messageId: string) => {
+    setBookmarks(prev => {
+      const next = prev.filter(b => b.messageId !== messageId);
+      saveBookmarks(next);
+      return next;
+    });
+  }, []);
+
+  const clearSession = useCallback((sessionKey: string) => {
+    setBookmarks(prev => {
+      const next = prev.filter(b => b.sessionKey !== sessionKey);
+      saveBookmarks(next);
+      return next;
+    });
+  }, []);
+
+  return { bookmarks, toggle, isBookmarked, getForSession, remove, clearSession };
+}