[Feature] Media and files: in-timeline preview without permanent download + consistent Media/Files gallery

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#### What would you like to do?

Preview files directly in the timeline and via **Room info -> Media and files** without requiring a permanent download. On Element X (Android + iOS): tap on image / video / PDF / text -> fullscreen native preview with swipe, Office (docx/xlsx/pptx) and zip -> system QuickLook / Open with on a temp cache (mxc:// buffered to sandbox, no Save to Files/Photos by default). Media tab = m.image / m.video / m.gallery, Files tab = m.file / m.audio. This is cross-platform (element-meta tracking issue).

#### Why would you like to do it?

- On Element Web, docx forces Download to disk; on Element X the file must be openable without that step.
- Users look for a recent image.png (m.image) in **Files**, but Element Web Files filters m.file only — expected that it appears in **Media** instead. On Element X phone it should be clearly in **Media** tab. Current confusion looks like a bug.
- Element X has no sidepanel/widget engine (element-meta#1915: No Integrations / Widgets / Matrix apps, No message search), so a custom sidepanel without forking Android+iOS is not possible. We want the native gallery/preview to cover the need.
- Covers room !JpfVweBAogOuHfYAEs:matrix.mettaway.voyage on matrix.mettaway.voyage with image.png (blob vector from @ardjo:matrix.org) and Current overview docx (17.7 KB) from @linalane:matrix.mettaway.voyage — verified 2026-08-17: timeline shows both inline, Room info -> Files lists only docx (image correctly in Media on X).

#### How would you like to achieve it?

Reuse existing native preview, no fork / no widget / no homeserver route:
- Android: libraries/mediaviewer/impl/.../LocalMediaView.kt:45-82 already dispatches image->MediaImageView, video->MediaVideoView, text->TextFileView, pdf->MediaPdfView (PdfRendererManager), audio->MediaAudioView, else->MediaFileView + Open with; cache via MatrixMediaLoader temp. Gallery via MediaGalleryState.kt / JoinedRustRoom.kt:198-217 (TimelineFilter.OnlyMessage).
- iOS: FilePreviewScreen/InteractiveQuickLook.swift / QLPreviewController, MediaEventsTimelineScreenCoordinator with allowedMessageTypes [.image,.video,.gallery] vs [.file,.audio,.gallery].
- Keep patch surface minimal: only MIME handling in LocalMediaView / InteractiveQuickLook if a gap is found during Phase 2 phone validation (tap in timeline + via Media and files, both tabs, pdf/txt without Save). No Synapse /last-30-days (client paginateBackwards + origin_server_ts filter), no LibreOffice conversion proxy.

Runbook: https://app.notion.com/p/3bf8ef36324d81ef9ee5d49f68bd8969

Start with Android's libraries/mediaviewer/impl/.../LocalMediaView.kt and MediaGalleryState.kt, then review JoinedRustRoom.kt:198-217; on iOS, read FilePreviewScreen/InteractiveQuickLook.swift and MediaEventsTimelineScreenCoordinator. Run the Phase 2 phone validation from the issue for timeline and Media and files previews, and consider the work done when the listed file types open from both tabs without a permanent download.

Written by the indexing model from the issue text.