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>

db/schema.sql

@@ -12,6 +12,8 @@ CREATE INDEX IF NOT EXISTS idx_raw_mails_address ON raw_mails(address);
 
 CREATE INDEX IF NOT EXISTS idx_raw_mails_created_at ON raw_mails(created_at);
 
+CREATE INDEX IF NOT EXISTS idx_raw_mails_message_id ON raw_mails(message_id);
+
 CREATE TABLE IF NOT EXISTS address (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     name TEXT UNIQUE,