fix(gallery): paginate media, activity log and export on detached timeline sets — never the live timeline (#163)

RoomTimeline renders a numeric index window into the live timeline's
event arrays; SDK back-pagination prepends, so any side panel calling
paginateEventTimeline(room.getLiveTimeline()) shifted the visible
messages into the past on the next render and broke at-bottom tracking.

New utils/detachedTimeline.ts builds a timeline set that mirrors the
already-loaded history and paginates independently: a room-registered
filtered set (server-side contains_url / types filter) when the filter
is usable, else a private EventTimelineSet seeded from the live timeline.
useRoomMediaTimeline wraps it for the gallery (live events + redactions
handled); RoomActivityLog uses a type filter (safe in encrypted rooms);
ExportRoomHistory pages a private set so a full export no longer parks
thousands of events in the live timeline.

Verified with Playwright against a local Synapse in a 400-message plain
room and a 200-message encrypted room: timeline stays at the bottom
through gallery pages, activity load-more and a full export; live
messages keep auto-scrolling; all media found in both rooms.

Also adds scripts/dev-homeserver.sh + scripts/dev-seed.py (local
throwaway Synapse for driving the real UI) and documents them.

Closes #163

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

LOTUS_FEATURES.md

@@ -1193,7 +1193,7 @@ When Lotus Chat is installed as a PWA (Android Chrome, desktop Chrome/Edge), the
 - **Files** — name/size/sender rows with download
 - **Jump to message** — a "Go to message" action on file rows, audio rows, and in the lightbox navigates the timeline to the source event (`useRoomNavigate`) and closes the drawer
 - Encrypted media is decrypted client-side on demand (no lock placeholder); download works for all types
-- **Auto-pagination** — an `IntersectionObserver` sentinel calls `mx.paginateEventTimeline()` to pull older media as you scroll (manual retry on error)
+- **Auto-pagination** — an `IntersectionObserver` sentinel pulls older media as you scroll (manual retry on error). Since Gitea #163 this pages through the gallery's **own timeline set** (`useRoomMediaTimeline` → `utils/detachedTimeline.ts`), never the room's live timeline: unencrypted rooms use a server-side `contains_url` filter (a page is 100 media events, not 100 events), encrypted rooms page raw history into a private set and filter after decrypting. The message list behind the drawer no longer jumps into the past; the Activity log and history Export use the same helper.
 
 ### Knock-to-Join