feat(android): export redesign, in/out marks on the timeline + export from playback (#617)

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Phase 1 of the Android export redesign (#617): bring Android's export flow in line with the desktop and iOS clients.

The problem

Android export was a stand-alone screen (feature/export/) whose only entry point was the Download icon on the playback wall. The single-camera playback screen had no export entry at all. The route took no arguments, so even the wall entry dropped the camera and moment being reviewed. The time range was editable solely by two [-] [+] steppers moving in one-minute steps, which made a second-accurate window impossible to express, even though the ViewModel already clamps to a 1 s minimum and the server accepts RFC-3339 with fractional seconds. There was also no way to stop a running job.

What this does

1. In/out selection on the playback timeline. PlaybackUiState gains raw exportSelStartMs / exportSelEndMs epoch millis. They are stored exactly as the operator placed them and ordered only when read, so dragging one handle past the other never swaps which handle the finger is holding, the same model as the desktop playback_timeline_controller (selStartMs/selEndMs, no snapping) and the iOS CenteredTimelineView. CenteredTimeline draws the bracket as a translucent amber region with a solid handle at each edge (mirrors iOS 2c/2e; new TimelineColors.exportFill/exportHandle in the existing amber family), and a press landing within grab range of a handle resizes that edge instead of scrubbing. Nearest handle wins on overlap, as on iOS.

Scrub and pinch behaviour is unchanged: the handle path returns from the gesture before onScrubStart() is ever called, and it never moves the playhead.

2. Touch-friendly marking. There is no Shift+drag on a phone, so Set in point / Set out point / Clear selection live with the other secondary actions (the 3-dot overflow in portrait, a "mark for export" menu on the transport row in landscape). Each places its end at the playhead and seeds the other end from the playhead, matching iOS setExportEdge; the timeline handles then fine-tune it.

3. "Export selection" bar. While a bracket exists, a bar under the video shows the exact start, end and duration to the second, with Clear and Export, mirroring the desktop's bar (same muted amber ground). The end drops its date when the range stays inside one day.

4. Seeded export route. Routes.EXPORT now takes optional cameraId, startMs, endMs, with a Routes.export(...) builder that percent-encodes the camera id (never form-encoding's +, which a URI reads as a literal plus). Playback passes the marked bracket, or the hour ending at the playhead when nothing is marked (iOS exportRange()); the wall passes the hour ending at its scrub cursor (the wall has no single focused camera, so it seeds the window only). ExportScreen opens pre-filled.

5. The one-minute steppers are gone. Each boundary is now a tappable second-accurate field backed by JumpToDateTimeDialog (new opt-in preserveSeconds, so opening the picker on a seeded selection does not silently round its seconds off), plus Last 1m / 5m / 10m / 15m quick chips (the desktop export builder's set) and a one-second nudge stepper on each side. start < end is enforced in the ViewModel and now gates Create.

6. Cancel a running job. DELETE /export/{job_id} is bound in CrumbApi/CrumbRepository (an unsuccessful status becomes a failure rather than a silent no-op) and surfaced as a Cancel button while the job is still stoppable. The poll loop also treats a server-side cancelled status as terminal, so it can't spin forever on one, and a deliberate cancel shows as a neutral notice rather than a red failure.

Selection arithmetic lives in one new pure object, feature/export/ExportRange.kt, shared by the timeline and the export screen so the two cannot drift.

Deferred to a follow-up (Phase 2, still under #617)

• Batch clip list via POST /export/batch (Android remains single-shot; desktop/iOS have the list).
• Output options: codec/container, audio, ZIP password.
• Filmstrip preview in the export builder.

Notes for review

• Models.kt's ExportJob/ExportOutputFile and ExportScreen's hardcoded .mp4 save names are deliberately untouched (a concurrent change owns those). The cancel path therefore reads status == "cancelled" through a local private extension rather than assuming a model-level flag, and composes with an isCancelled property landing later.
• Docs updated in the same change: docs/COMPONENT-MAP.md export row, docs-site/docs/playback/export.md (new "Marking the moment you want" section) and docs-site/docs/clients/android.md.

Verification

Verified: compile + unit tests green on dev2 (:app:compileDebugKotlin testDebugUnitTest, BUILD SUCCESSFUL); 27 new unit tests cover selection normalization, edge seeding/clamping/future-clamping, the export-range fallback, range validation, quick-chip math and route arg encoding. On-device visual check recommended (bracket rendering, handle drag vs. scrub, the selection bar in both orientations).

Part of #617

[badbread]

Rebased onto #620: this branch now carries #620's commit (c759630) via merge 82ab281, so it merges cleanly after #620 (or merging this alone brings both; GitHub will mark #620 merged). Merged branch re-gated on dev2: compileDebugKotlin + testDebugUnitTest green. A combined debug APK (main + #620 + #621) was built from 82ab281 for on-device verification.