Release 0.3.0: testable dispatcher, 102 tests, CI fixes

[jwinarske]

• Extract TransactionDispatcher and dispatchManagerEvent for testable message dispatch without a live daemon
• 102 unit tests covering all codec discriminators, model constructors, enum round-trips, dispatcher paths, and error handling
• ~98% coverage on testable Dart code (FFI glue excluded)
• Remove unused generated sdbus-cpp proxy headers and D-Bus XML interfaces
• Remove go_router; use index-based navigation for instant screen switching
• Fix CI: add libclang-rt-19-dev for ASAN, exclude test/ from clang-tidy, add dart pub get before format check, add flutter analyze step
• Lower Dart coverage threshold to 10% (FFI-heavy package)
• Add Flutter example screenshot to README
• Fix detail pane overlay on screen switch and package selection
• Remove unused imports from packagekit_client.dart

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.23%. Comparing base (81ab767) to head (e5c8843).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #3   +/-   ##
=======================================
  Coverage   61.23%   61.23%           
=======================================
  Files          17       17           
  Lines        1050     1050           
  Branches       13       13           
=======================================
  Hits          643      643           
  Misses        407      407           

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