feat: add message search and conversation previews

[chenyme]

Summary

Improve conversation search so users can find more than the latest few conversations and inspect results before opening them.

• Search conversation titles, message content, projects, labels, and models.
• Add paginated loading with infinite scrolling.
• Add a desktop split-view preview that lazily loads the latest 10 messages.
• Render preview messages with Streamdown.
• Add keyboard navigation and persistent compact/preview display modes.
• Keep mobile and compact modes lightweight by skipping preview requests.
• Constrain both panes to independent scrolling and animate dialog size changes.
• Synchronize Swagger and generated frontend API contracts.

Change type

• Bug fix
• Feature
• Documentation
• Refactor
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

• pnpm --filter @deeix/web check
• pnpm --filter @deeix/web build
• git diff --check HEAD

Screenshots, API examples, or logs

Added desktop split-view conversation search with lazy message previews. On mobile or in compact mode, search remains list-only and does not request preview messages.

Configuration, migration, and compatibility notes

No configuration or database migration is required.

The conversation search API changes are additive. Swagger and the generated frontend API contract have been updated together.

The selected desktop display mode is stored locally under deeix.navigation-search.preview.open.

Documentation

• Documentation is not needed for this change.
• Documentation was updated.
• Documentation still needs to be updated.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• Conversation search and preview access remain scoped to the authenticated user.
• Search input, pagination parameters, and conversation ownership checks were reviewed.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior and API contract changes are documented in this PR.
• Generated artifacts are included only where required for Swagger and API contracts.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.