feat(media): consecutive photos/videos render as one gallery grid (#137)

Client-side only: every file is still its own standard m.image/m.video
event, so Element and friends keep seeing N plain images. In Lotus a run
of media from one sender — contiguous, ≤ 60 s apart, no reply/thread/edit
relation, up to 10 — renders once, at its last event, as a 2–4 column
grid of square thumbnails (blurhash placeholder, video play badge,
tap-to-load when media auto-load is off). A member with reactions or a
thread closes its group so those stay visible under the rendered event.

Tapping a tile opens the lightbox on just that group in send order
(←/→, zoom, download, jump). "Show separately" splits a group back into
individual messages for the session; "Show as gallery" undoes it.

Planning is lazy per render pass (utils/mediaGroups.ts, unit-tested):
the first media event met plans its whole run in both directions, so a
virtual window that starts mid-run agrees with one that starts before it.

Verified: 5 files dropped at once in an encrypted room — both sender and
recipient see one 5-tile grid with decrypted thumbnails; desktop + phone;
a reaction on photo 3 yields [1–3]+👍 and [4–5].

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

src/app/features/room/MediaGallery.tsx

@@ -58,7 +58,7 @@ const TAB_MSGTYPES: Record<GalleryTab, MsgType> = {
 
 type DecryptState = { status: 'loading' } | { status: 'ok'; url: string } | { status: 'error' };
 
-function useDecryptedMediaUrl(
+export function useDecryptedMediaUrl(
   mx: MatrixClient,
   mxcUrl: string | undefined,
   encInfo: IEncryptedFile | undefined,
@@ -154,7 +154,7 @@ function getSenderName(room: Room, userId: string): string {
 // the grid and the lightbox must use this so their positional indices stay in
 // lockstep — otherwise a tile skipped for lack of a thumb would shift the
 // lightbox and open the wrong media.
-function getThumbMxc(mEvent: MatrixEvent): string | undefined {
+export function getThumbMxc(mEvent: MatrixEvent): string | undefined {
   const c = mEvent.getContent();
   const isEnc = !!c.file;
   const info: (IImageInfo & IThumbnailContent) | undefined = c.info;