feat: improve composer mentions and connector account selection

[AnishSarkar22]

Description

• Added a cleaner @ mention picker for documents, folders, SurfSense docs, and connected accounts.
• Improved / prompt suggestions with smoother loading states and more consistent popup behavior.
• Added recent mention support so users can quickly reuse recently selected context.
• Improved mention chips and composer styling for a more polished chat experience.
• Added support for selecting a specific connector account, so the agent can use the intended account when tools need one.

Motivation and Context

FIX #

Screenshots

API Changes

• This PR includes API changes

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactoring
• Documentation
• Dependency/Build system
• Breaking change
• Other (specify):

Testing Performed

• Tested locally
• Manual/QA verification

Checklist

• Follows project coding standards and conventions
• Documentation updated as needed
• Dependencies updated as needed
• No lint/build errors or new warnings
• All relevant tests are passing

High-level PR Summary

This PR extends the chat composer's @-mention system to support connector account selection alongside the existing documents and folders. Users can now explicitly select which connected account (e.g., specific Google Drive or Notion account) to use when mentioning context, allowing the backend to route tool calls to the exact account the user intended. The frontend composer suggestion popup has been redesigned with a hierarchical browse UI (root → category → items) and supports recent mentions. The backend accepts connector mentions as separate payload fields (mentioned_connector_ids and mentioned_connectors) and embeds them in the LLM context so tools can prefer the selected account when a connector_id parameter is available. The changes include schema updates to support a three-way discriminated union (doc, folder, connector), UI components for the new suggestion popover with keyboard navigation, and recents tracking in localStorage.

⏱️ Estimated Review Time: 30-90 minutes

💡 Review Order Suggestion

Order	File Path
1	surfsense_backend/app/schemas/new_chat.py
2	surfsense_backend/app/agents/new_chat/context.py
3	surfsense_web/atoms/chat/mentioned-documents.atom.ts
4	surfsense_web/lib/chat/mention-doc-key.ts
5	surfsense_web/components/new-chat/composer-suggestion-popup.tsx
6	surfsense_web/components/new-chat/use-composer-suggestion-navigator.ts
7	surfsense_web/components/assistant-ui/inline-mention-editor.tsx
8	surfsense_web/components/new-chat/document-mention-picker.tsx
9	surfsense_web/components/assistant-ui/thread.tsx
10	surfsense_backend/app/agents/new_chat/mention_resolver.py
11	surfsense_backend/app/routes/new_chat_routes.py
12	surfsense_backend/app/tasks/chat/persistence.py
13	surfsense_backend/app/tasks/chat/stream_new_chat.py
14	surfsense_web/app/dashboard/[search_space_id]/new-chat/[[...chat_id]]/page.tsx
15	surfsense_web/components/assistant-ui/user-message.tsx
16	surfsense_web/components/assistant-ui/mention-chip.tsx
17	surfsense_web/components/assistant-ui/connector-popup/constants/connector-constants.ts
18	surfsense_web/components/new-chat/prompt-picker.tsx
19	surfsense_web/components/documents/FolderTreeView.tsx
20	surfsense_web/components/layout/ui/sidebar/DocumentsSidebar.tsx
21	surfsense_web/components/auth/sign-in-button.tsx
22	surfsense_web/lib/connector-telemetry.ts

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