fix(android): treat cancelled exports as terminal; honor server output filename

[badbread]

Summary

Two small, verified bugs in the Android export flow (#619).

1. A cancelled export job polls forever. The server's DELETE /export/{job_id} sets status = "cancelled" and the poller is documented to stop on that one terminal state. ExportJob.isTerminal in data/Models.kt only checked isDone || isFailed, so ExportViewModel.startPolling never noticed a cancelled job and kept polling indefinitely, with the UI stuck showing a spinner. Fix: add ExportJob.isCancelled and fold it into isTerminal; the poller now stops without an error toast (cancellation is user-initiated, not a failure), and ExportScreen's status label shows "Cancelled".

2. Exported files are saved with the wrong extension. ExportOutputFile.filename on the server carries the real on-disk basename (.mkv for a no-transcode passthrough, or a whole-job .zip), but the Android model never decoded that field, and the save/share code hardcoded a crumb-export-<id>....mp4 name plus a video/mp4 MIME type regardless of the actual container. An .mkv or .zip export was saved/shared as .mp4, producing an unplayable/corrupt-looking file. Fix: decode filename, derive the extension (and matching MIME type for MediaStore / the share intent) from it at both save sites (Downloads save, and cache-then-share), falling back to mp4 only when the server didn't report a filename.

Files changed

• apps/android/app/src/main/java/video/crumb/app/data/Models.kt: ExportOutputFile.filename, ExportJob.isCancelled/isTerminal.
• apps/android/app/src/main/java/video/crumb/app/feature/export/ExportViewModel.kt: terminal-but-not-failed handling for cancelled jobs.
• apps/android/app/src/main/java/video/crumb/app/feature/export/ExportScreen.kt: "Cancelled" status label, exportFileExtension/exportMimeType helpers used by saveExportToDownloads, downloadExportFileToCache, and shareLocalFile.

Test plan

• Verified: ./gradlew :app:compileDebugKotlin testDebugUnitTest green on dev2 (BUILD SUCCESSFUL, exit 0).
• Manual on-device check: cancel an in-progress export and confirm polling stops with a "Cancelled" status; download an .mkv-source export and confirm the saved file keeps the .mkv extension.

Fixes #619