feat: optimize email filtering with frontend-only search (#787)

* feat: optimize email filtering with frontend-only search

- Remove backend keyword parameter from mail APIs (breaking change)
- Implement frontend filtering on current page (20-100 items)
- Add message_id database index for UPDATE performance
- Support desktop and mobile responsive layouts
- Update API documentation and CHANGELOG

BREAKING CHANGE: /admin/mails and /user_api/mails no longer accept keyword parameter

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

* fix: restore Mail ID query input in Index.vue

- Keep showMailIdQuery UI input for querying specific mail by ID
- Triggered when URL contains mail_id parameter

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/package.json

@@ -40,7 +40,7 @@
   "devDependencies": {
     "@vicons/fa": "^0.13.0",
     "@vicons/material": "^0.13.0",
-    "@vitejs/plugin-vue": "^6.0.2",
+    "@vitejs/plugin-vue": "^6.0.3",
     "unplugin-auto-import": "^20.3.0",
     "unplugin-vue-components": "^30.0.0",
     "vite": "^7.2.7",