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

src/app/features/room-settings/ExportRoomHistory.tsx

@@ -4,6 +4,7 @@ import { EventType } from 'matrix-js-sdk';
 import { Page, PageContent, PageHeader } from '../../components/page';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { useRoom } from '../../hooks/useRoom';
+import { createDetachedTimelineSet } from '../../utils/detachedTimeline';
 import { useRoomName } from '../../hooks/useRoomMeta';
 import { SequenceCard } from '../../components/sequence-card';
 import { SequenceCardStyle } from '../common-settings/styles.css';
@@ -73,7 +74,10 @@ export function ExportRoomHistory({ requestClose }: ExportRoomHistoryProps) {
       // so we must deduplicate by eventId to avoid re-adding the same events
       // on each pagination step.
       const seen = new Set<string>();
-      const timeline = room.getLiveTimeline();
+      // [Gitea #163] Paginate a private timeline set, not the live one: the
+      // export can pull thousands of events and the room behind this modal
+      // renders the live timeline by index.
+      const timeline = createDetachedTimelineSet(mx, room).set.getLiveTimeline();
       let canLoadMore = true;
       // Track the oldest collected timestamp incrementally so the fromTs check
       // doesn't rescan the whole `collected` array on every pagination step.