feat: add theme provider and control components for light/dark mode

- Implemented ThemeProvider to manage theme state and preferences.
- Added ThemeControl component for users to switch between system, light, and dark themes.
- Integrated localStorage for theme preference persistence.
- Updated document styles based on theme changes.

test: add tests for permanent deletion of habits and related records

- Created tests to ensure permanent deletion of habits removes all dependent records.
- Verified that deletion requires ownership, authentication, and checks for archived habits.
- Added tests to confirm rollback behavior on deletion failures.

test: add tests for habit palette and progress card functionality

- Implemented tests for habit color progression and theme-based color shading.
- Added tests for progress card calculations and calendar display logic.

feat: define theme types and preferences

- Introduced Theme and ThemePreference types for better type safety.
- Created utility functions for resolving theme based on user preference and system settings.
- Defined theme palettes for light and dark modes to ensure adequate contrast.

docs/API.md

@@ -20,6 +20,7 @@ Run `bun dev` to migrate the local database and serve the API. Run `bun run test
 | GET | `/habits/:id` | Current habit configuration and revision ID |
 | PATCH | `/habits/:id` | Change name, method, method settings, schedule or archived flag |
 | DELETE | `/habits/:id` | Archive from today, preserving history; 204 |
+| DELETE | `/habits/:id/permanent` | Permanently delete an owned archived habit and all its history; 204. Active habits return 409. Remove it from combined charts and delete charts left empty. |
 | GET | `/habits/:id/history` | `{revisions: [...]}` newest first, including superseded same-day revisions |
 | GET | `/habits/:id/tasks` | `{tasks: [...]}` for current task method |
 | POST | `/habits/:id/tasks` | Add task; 201 |