fix: cannot select directory hangs on “Moving data” (Android) (#484)

[harikrishna-au]

Description

This PR fixes a bug where changing the data directory could hang on the "Moving data to new directory" screen and silently fall back to the default directory.

Key Changes

• Guard SAF (Storage Access Framework) destinations
  Detect non-file-system targets and return a clear “not permitted” result instead of attempting a move that would hang/fail.
• Skip Flutter runtime assets
  Avoid copying flutter_assets/kernel_blob.bin and other runtime files to prevent illegal copies.
• Use context-free dialogs/snackbar via GetX
  Prevents use_build_context_synchronously issues after await calls.
• Add success feedback
  Shows confirmation when a move completes successfully.
• Android: enableOnBackInvokedCallback in Manifest
  Aligns with Android 13+ back behavior.

Behavior Notes

• Moving to SAF-picked folders (content:// URIs) is now blocked with a friendly error.
  Full SAF directory write support can be added later using existing SAF utilities.
• Moving to regular file-system directories works and shows success feedback.

Manual Test Plan

1. Normal on-device directory
   • Move completes successfully
   • “Current directory” updates
   • Tasks persist in the new directory
2. SAF tree (e.g., Downloads via system picker)
   • Operation is rejected with a clear message
   • No hang and no partial move
3. Ensure no Flutter runtime files are copied.

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Dependencies

None

Fixes

Fixes #484

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Screenshots

N/A (UI unchanged apart from dialogs/snackbar)

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Checklist

• Tests have been added or updated to cover the changes
• Documentation has been updated to reflect the changes
• Code follows the established coding style guidelines
• All tests are passing

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Notes:
Tests for SAF behavior and settings controller can be added in a follow-up PR (current CI doesn’t run tests).

[harikrishna-au]

Screen.Recording.2025-08-15.at.12.40.21.AM.mp4

@JRadl @SGI-CAPP-AT2 review

[harikrishna-au]

@SGI-CAPP-AT2, is there anything missed?