Fix warnings, port deprecated GTK/libadwaita APIs, rework benchmark, update april-asr

[Techlm77]

This fixes compiler warnings, ports away from deprecated GTK and libadwaita APIs, reworks the setup benchmark, and updates april-asr with streaming and accuracy fixes. Each commit builds on its own.

Depends on abb128/april-asr#23. The submodule currently points at Techlm77/april-asr so this branch builds. Once #23 is merged I'll switch .gitmodules back to abb128/april-asr and bump the commit, or feel free to do that when merging.

Commits

1. Fix compiler warnings: unused parameters, an unused audio thread function, size_t-correct comparisons and format specifiers, and handling APRIL_RESULT_UNKNOWN.
2. Port away from deprecated GTK and libadwaita APIs:
   • Preferences use AdwPreferencesDialog, presented without a parent so they still open in their own window rather than inside the small captions window.
   • About uses AdwAboutDialog, alerts use AdwAlertDialog, and file choosers use GtkFileDialog.
   • URLs open with GtkUriLauncher and GtkFileLauncher. CSS loads with load_from_string through a display provider.
   • G_APPLICATION_DEFAULT_FLAGS replaces G_APPLICATION_FLAGS_NONE.
   • The gdk_x11_* keep-above accessors have no replacement, so their warnings are silenced around that one function only.
   • Requires GTK 4.12 and libadwaita 1.5.
3. Rework the setup benchmark: this builds on e69209d.
   • Timing uses the monotonic clock, with an untimed warm-up and a final flush.
   • Audio goes in 100 ms chunks, so the progress bar is smooth and ends full.
   • It stops early once a pass is impossible.
   • Results reach the main thread by value instead of through volatile fields.
   • Closing the window mid-benchmark previously could crash. The thread is now stopped and joined.
4. Update april-asr:
   • The engine fixes are in Fix streaming correctness and feature extraction april-asr#23. Captions finalize after 1.2 s of silence instead of 2.2 s.
   • The session switches to APRIL_CONFIG_FLAG_ASYNC_NO_RT_BIT, so audio is no longer time-compressed when the CPU falls behind, which hurt accuracy. The trade-off is that a machine too slow for real time gets CANT_KEEP_UP errors instead of degraded text. Happy to drop this if you prefer the old behaviour.
   • C++ is enabled in Meson because april-asr's CMake project now builds its C++17 examples.

Testing

• Build: a fresh clone builds with no compiler warnings in the app or the engine.
• Accuracy: on 80 LibriSpeech clips (1,601 words) with the bundled English model, word errors went from 215 to 201 (13.4% → 12.6%). Details are in Fix streaming correctness and feature extraction april-asr#23.
• Run on Arch Linux (KDE): the app has been launched and run with these changes.
• Not yet hand-tested in the new dialogs: keep-on-top for Preferences on X11, the model load error dialog, and history export and erase.

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