Merges FN-5218: adds hash mention support (`#`) in chat composers, grouping task and file reference results in a unified popup, with wiring into `ChatView` and `QuickChatFAB`, plus test coverage and docs. The hook `useFileMention` is extended and refactored, and the file mention popup UI is updated
Fusion-Task-Id: FN-5218
- Add file search API wiring in dashboard client/server with route coverage for /api/files/search
- Introduce useFileMention hook and FileMentionPopup component, integrated into ChatView and QuickChatFAB
- Resolve @file references in chat messages so referenced file content is included in AI context
- Add styling, mobile responsiveness, and focused tests for popup UI, hook behavior, chat resolution, and file services
