refactor: convert password change to Server Action

- Add src/app/actions/password.ts with changePassword
- Update ChangePasswordForm to use Server Action
- Remove api/user/password route (entire file)

docs/server-actions-plan.md

@@ -13,6 +13,7 @@
 | `PUT /api/preferences` | `updatePreferences()` | ✅ Done |
 | `POST /api/komga/libraries/[libraryId]/scan` | `scanLibrary()` | ✅ Done |
 | `POST /api/komga/config` | `saveKomgaConfig()` | ✅ Done |
+| `PUT /api/user/password` | `changePassword()` | ✅ Done |
 
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