Remove unused usePlatformService composable and document mixin architecture

- Delete unused usePlatformService.ts file
- Create architecture-decisions.md documenting mixin choice over composables
- Update README.md with architecture decision reference and usage guidance
- Document rationale: performance, consistency, developer experience
- Maintain established class-based component pattern with vue-facing-decorator

README.md

@@ -88,18 +88,23 @@ Key principles:
 
 ### Database Architecture
 
-The application uses a platform-agnostic database layer:
+The application uses a platform-agnostic database layer with Vue mixins for service access:
 
 * `src/services/PlatformService.ts` - Database interface definition
 * `src/services/PlatformServiceFactory.ts` - Platform-specific service factory
 * `src/services/AbsurdSqlDatabaseService.ts` - SQLite implementation
+* `src/utils/PlatformServiceMixin.ts` - Vue mixin for database access with caching
 * `src/db/` - Legacy Dexie database (migration in progress)
 
 **Development Guidelines**:
-- Always use `PlatformService` for database operations
+
+- Always use `PlatformServiceMixin` for database operations in components
 - Never import Dexie directly in application code
 - Test with `USE_DEXIE_DB = false` for new features
 - Use migration tools for data transfer between systems
+- Leverage mixin's ultra-concise methods: `$db()`, `$exec()`, `$one()`, `$contacts()`, `$settings()`
+
+**Architecture Decision**: The project uses Vue mixins over Composition API composables for platform service access. See [Architecture Decisions](doc/architecture-decisions.md) for detailed rationale.
 
 ### Kudos