fix: resolve Flutter 3.32.0 compatibility issues

- Update all CI workflows to use Flutter 3.32.0 consistently
- Fix DropdownButtonFormField to use 'value' instead of 'initialValue' for Flutter 3.32.0
- Update fl_chart to 0.69.0 for compatibility with Flutter 3.32.0
- Fixes CI build failures due to version mismatches

fix: replace initialValue with value in DropdownButtonFormField widgets

- Fixed Flutter SDK compatibility issue where initialValue parameter was removed
- Updated all DropdownButtonFormField widgets to use value parameter instead
- Affected files: assignments, calendar, classes, and grades screens

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Co-Authored-By: Claude <noreply@anthropic.com>

feat: add unified desktop release workflow

- Combines Windows and macOS builds in parallel
- Creates single release with both DMG and ZIP
- Uses single version tag (v*) for both platforms

fix: update macOS workflow to match Windows convention

- Rename to 'MacOS Release' for consistency
- Remove trigger on main branch pushes (wasteful)
- Keep PR builds for validation
- Only create releases on tags or manual trigger

Merge pull request #42 from Sallvainian/fix-dart-syntax-errors

Fix Dart syntax errors in desktop OAuth handler

fix: Update Windows release workflow to trigger on workflow_dispatch

- Fixed create-release job condition to trigger on both tag pushes and manual workflow_dispatch
- Added dynamic tag_name generation for manual triggers (windows-v{version})
- Ensures GitHub Release is created regardless of trigger method

fix: Improve iOS build robustness

- Add iOS 18 SDK check and conditional download
- Make GoogleService-Info.plist check non-fatal with debugging
- Check directory structure to diagnose file location issues
- Install iOS platform if SDK not available