[Feat] Chat : Resumable streaming across in-app navigation + optional message timestamps

[CREDO23]

Summary

• Resumable streaming: moves the chat stream engine out of the (unmounting) chat page into a module-level store + a persistent runner in the workspace shell, so an in-flight turn keeps streaming across in-app navigation and reattaches live on return. Single active stream, keyed by threadId. Scope: in-app (client-side) navigation only — full page reload, tab close, and cross-device are out of scope. Frontend-only, no backend changes.
• Message timestamps: opt-in per-message timestamps under user & assistant messages, off by default (localStorage), toggled from a new User Settings → Appearance tab. Surfaces timestamp data that already existed end-to-end.

High-level PR Summary

This PR introduces resumable streaming across in-app navigation and optional message timestamps to the chat interface. The streaming refactor moves the chat stream engine from the chat page into a module-level store (chatStreamStore) with a persistent runner in the workspace shell, allowing in-flight chat turns to continue streaming when users navigate between workspace routes and seamlessly reattach when they return. This is scoped to client-side navigation only (excludes full page reloads, tab closes, and cross-device scenarios). The message timestamps feature adds an opt-in per-message timestamp display (off by default, controlled via localStorage) through a new User Settings → Appearance tab, surfacing timestamp data that already existed end-to-end. Both features are frontend-only with no backend changes.

⏱️ Estimated Review Time: 1-3 hours

💡 Review Order Suggestion

Order	File Path
1	surfsense_web/lib/chat/stream-engine/store.ts
2	surfsense_web/lib/chat/stream-engine/helpers.ts
3	surfsense_web/lib/chat/stream-engine/use-chat-stream.ts
4	surfsense_web/components/chat/active-chat-stream-runner.tsx
5	surfsense_web/lib/chat/stream-engine/engine.ts
6	surfsense_web/app/dashboard/[workspace_id]/new-chat/[[...chat_id]]/page.tsx
7	surfsense_web/components/layout/providers/LayoutDataProvider.tsx
8	surfsense_web/atoms/chat/show-timestamps.atom.ts
9	surfsense_web/lib/format-date.ts
10	surfsense_web/components/assistant-ui/message-timestamp.tsx
11	surfsense_web/components/assistant-ui/user-message.tsx
12	surfsense_web/components/assistant-ui/assistant-message.tsx
13	surfsense_web/app/dashboard/[workspace_id]/user-settings/components/AppearanceContent.tsx
14	surfsense_web/app/dashboard/[workspace_id]/user-settings/layout-shell.tsx
15	surfsense_web/app/dashboard/[workspace_id]/user-settings/appearance/page.tsx

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