feat: Add Chat UX improvements with notifications and @mention support

- Add ActionBar component with expandable toolbar for mobile
- Add @mention functionality with autocomplete dropdown
- Add browser notification system (push, sound, vibration)
- Add NotificationSettings modal for user preferences
- Add mention badges on room list cards
- Add ReportPreview with Markdown rendering and copy/download
- Add message copy functionality with hover actions
- Add backend mentions field to messages with Alembic migration
- Add lots field to rooms, remove templates
- Optimize WebSocket database session handling
- Various UX polish (animations, accessibility)

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

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

openspec/specs/ai-report-generation/spec.md

@@ -262,3 +262,91 @@ The system SHALL track report generation status and notify users of completion v
   ```
 - **AND** the error message SHALL be user-friendly (no technical details)
 
+### Requirement: DIFY Service Health Check
+
+The system SHALL provide a health check mechanism to verify DIFY AI service connectivity and configuration.
+
+#### Scenario: Check DIFY configuration on startup
+- **WHEN** the application starts
+- **AND** `DIFY_API_KEY` is not configured
+- **THEN** the system SHALL log a warning message: "DIFY_API_KEY not configured - AI report generation will be unavailable"
+
+#### Scenario: DIFY health check endpoint
+- **WHEN** a user sends `GET /api/reports/health`
+- **AND** `DIFY_API_KEY` is not configured
+- **THEN** the system SHALL return:
+  ```json
+  {
+    "status": "error",
+    "message": "DIFY_API_KEY 未設定，請聯繫系統管理員"
+  }
+  ```
+
+#### Scenario: DIFY service unreachable
+- **WHEN** a user sends `GET /api/reports/health`
+- **AND** `DIFY_API_KEY` is configured
+- **BUT** the DIFY service cannot be reached
+- **THEN** the system SHALL return:
+  ```json
+  {
+    "status": "error",
+    "message": "無法連接 AI 服務，請稍後再試"
+  }
+  ```
+
+### Requirement: Report Generation Status Polling
+
+The frontend SHALL implement polling mechanism to ensure report status updates are received even if WebSocket connection is unstable.
+
+#### Scenario: Poll report status after generation trigger
+- **WHEN** a user triggers report generation
+- **AND** receives the initial `report_id`
+- **THEN** the frontend SHALL poll `GET /api/rooms/{room_id}/reports/{report_id}` every 2 seconds
+- **AND** continue polling until status is "completed" or "failed"
+- **AND** timeout after 120 seconds with user-friendly error message
+
+#### Scenario: Display generation progress
+- **WHEN** polling returns status "collecting_data"
+- **THEN** the UI SHALL display "正在收集聊天室資料..."
+- **WHEN** polling returns status "generating_content"
+- **THEN** the UI SHALL display "AI 正在分析並生成報告內容..."
+- **WHEN** polling returns status "assembling_document"
+- **THEN** the UI SHALL display "正在組裝報告文件..."
+
+#### Scenario: Display generation error
+- **WHEN** polling returns status "failed"
+- **THEN** the UI SHALL display the `error_message` from the response
+- **AND** provide option to retry generation
+
+### Requirement: Markdown Report Output
+The report generation system SHALL provide reports in Markdown format for in-page preview.
+
+#### Scenario: Get report as Markdown
+- **WHEN** user requests `GET /api/rooms/{room_id}/reports/{report_id}/markdown`
+- **AND** the report status is `completed`
+- **THEN** the system returns the report content in Markdown format
+- **AND** the Markdown includes all report sections (summary, timeline, participants, etc.)
+
+#### Scenario: Markdown includes metadata
+- **WHEN** generating Markdown output
+- **THEN** the output includes a metadata header with room info, LOT numbers, dates
+- **AND** the format is suitable for copy-paste to other platforms
+
+### Requirement: In-Page Report Preview
+The frontend SHALL display a preview of the generated report within the chat room interface.
+
+#### Scenario: Display report preview
+- **WHEN** user clicks on a completed report
+- **THEN** a modal or drawer opens showing the Markdown-rendered report
+- **AND** the preview includes proper formatting (headers, tables, lists)
+
+#### Scenario: Copy Markdown content
+- **WHEN** user clicks "Copy Markdown" in the preview
+- **THEN** the raw Markdown text is copied to clipboard
+- **AND** a success toast notification is shown
+
+#### Scenario: Download Word from preview
+- **WHEN** user clicks "Download Word" in the preview
+- **THEN** the .docx file is downloaded
+- **AND** the filename uses the report title
+