Update the GUI to use flet

[MitchellThompkins]

PR Description: Merge update-python-rclone-dep and GUI Improvements

This PR merges the changes from the update-python-rclone-dep branch into new-gui, resolves merge conflicts, and ensures the CLI/GUI is fully functional with the new dependency structure.

Changes

1. Merge Conflict Resolution

• pyproject.toml & src/rclone_decrypt/__init__.py: Maintained version 0.2.0 as the base for the new GUI features.
• src/rclone_decrypt/decrypt.py:
  • Integrated dynamic rclone configuration discovery using rclone config file.
  • Implemented a more robust subprocess-based rclone copy execution, removing the direct rclone Python library dependency for core decryption tasks.
  • Preserved atomic move optimizations by creating temporary directories in the same parent folder as the target files.
  • Fixed linting issues (E501 line length) by cleaning up verbose merge comments.

2. CLI & GUI Integration

• src/rclone_decrypt/cli.py:
  • Re-introduced the --gui flag to launch the Flet-based interface.
  • Set --config to be optional when launching the GUI, as the interface provides its own config picker.
• src/rclone_decrypt/gui.py:
  • Updated UI titles to "rclone-decrypt" for consistency.

3. Dependencies & Tooling

• poetry.lock: Updated to reflect the merged dependencies and versioning.
• Linting: Ensured the codebase passes flake8 checks.

Verification Results

Automated Tests

• Ran poetry run pytest.
• Result: 9 passed (100% success rate).

Linting

• Ran poetry run flake8.
• Result: Clean (No errors).

CLI Verification

• Verified poetry run rclone-decrypt --help displays the new --gui option correctly.