chore: remove Flutter GUI entirely, Tauri is the sole frontend

Flutter (apps/flutter/) was at feature parity but maintaining two GUI
frontends doubles effort. Consolidate on Tauri v2 which handles both
desktop and mobile. Remove all Flutter source, config, and documentation
references.

PLAN.md

@@ -768,7 +768,11 @@ All Android work can be done locally on Linux. iOS must go through CI or a Mac.
 
 ---
 
-### Known Blockers
+### Tauri GUI
+
+Tauri v2 has mobile support but it's newer and less mature.
+
+#### Known Blockers
 
 **`notify` crate doesn't compile for mobile.** The file-watcher subsystem (`notify` + `notify-debouncer-mini` in `Cargo.toml`) does not support Android or iOS targets. The entire file-watcher initialization path must be gated behind `#[cfg(not(mobile))]` before cross-compilation will succeed.
 

README.md

@@ -198,7 +198,7 @@ cargo test -- --nocapture
 
 ## What's Next?
 
-- **Phase 4**: Mobile support (iOS & Android via Tauri v2)
+- **Phase 4**: Mobile support (iOS & Android via Tauri v2 mobile)
 - **Phase 5**: GUI advanced features (subtasks, search, date picker)
 - **Phase 6**: Mobile polish and platform-specific integrations
 - **Phase 7**: Google Tasks importer and unique features