feat(bookmarks): add message bookmarks (MSC4438)

[Just-Insane]

Description

Implements MSC4438 message bookmarks as an experimental feature, allowing users to save individual messages to a personal bookmark list stored in Matrix account data.

Fixes #600

What's included:

• Data layer: bookmarkDomain.ts, bookmarkRepository.ts — bookmark ID algorithm (djb2-style, compatible with Cinny reference impl), account data CRUD helpers following MSC4438 ordering guarantees (item written before index)
• State: state/bookmarks.ts — Jotai atoms (bookmarkListAtom, bookmarkLoadingAtom, bookmarkIdSetAtom) reactive to Matrix account data
• Hook: useBookmarks.ts — useBookmarkList, useIsBookmarked, useBookmarkActions (refresh, add, remove with optimistic updates)
• UI — Bookmarks page: BookmarksList.tsx — sidebar page with room filter, Jump button, remove button
• UI — Message menu item: MessageBookmarkItem in Message.tsx — context menu entry to bookmark/unbookmark a message
• Routing: new bookmarks path under home, useHomeBookmarksSelected hook, wired into Router.tsx
• Experimental settings toggle: MSC4438MessageBookmarks.tsx in Experimental settings — per-user opt-in
• Tests: unit tests for domain logic, repository layer, and Jotai atoms (74 tests)

The feature is gated by two independent mechanisms (either enables it): the config.json experiments.messageBookmarks flag (operator level) and the per-user experimental settings toggle.

References:

• MSC: MSC4438: Message bookmarks via account data matrix-org/matrix-spec-proposals#4438
• Docs PR: docs(polls): add polls feature documentation docs#12

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

This PR was fully generated with GitHub Copilot. The implementation adds MSC4438 message bookmarks: a djb2-style hash function generates a stable bookmark ID from roomId|eventId, which is used as the account data event-type suffix. The repository layer writes item events before updating the index (as required by MSC4438 for cross-device consistency) and soft-deletes items on removal since account data cannot be deleted from the server. Jotai atoms hold the cached bookmark list and a derived Set for O(1) per-message lookup. The UI consists of a sidebar list page (with room filter and jump/remove actions) and a context menu entry on message bubbles that toggles bookmark state with optimistic local updates.