Add dedicated export progress page with cancel support

[LukeNeedham]

Summary

Implements the top item from TODO.md:

on the create export page, once the user clicks to create the export, open a new page that shows progress and also a cancel button. when progress completes navigate automatically to the video view page

• Clicking Export on the create-export page now navigates to a new dedicated progress page instead of showing progress inline on the create page.
• The new page shows a progress bar and a Cancel button. Cancelling stops the underlying Media3 Transformer and deletes the partial output file, then returns to the create page.
• On success, the app automatically navigates to the exported video view page (replacing the progress page in the back stack, so back navigation returns to the create page, matching prior behaviour).
• The export parameters (selected videos, date range, include-date-stamp, name) are now passed between pages via a new ExportRequest model.
• ExportDiaryCreateViewModel no longer performs the export itself — that responsibility moved to the new ExportDiaryProgressViewModel, which owns the export/cancel/save-export flow.

Test plan

• ./gradlew assembleDebug builds successfully
• ./gradlew test passes
• Manually verify: create export → progress page shows → cancel returns to create page with a clean partial-file removal
• Manually verify: create export → progress page shows → completes → auto-navigates to video view page
• Manually verify: back button from video view page returns to create page (not progress page)

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Generated by Claude Code

[github-actions]

App apk at:
https://github.com/LukeNeedham/VideoDiary/releases/download/claude-top-todo-p904d9-123-1/app-debug.apk

• Triggered by: PR Commit 39afa702bbd6e6e4eb2acbbbcc5b082f7a2f5d74
• Built at: 05/08/2026 11:43:26