fix(web-ui): nav search sessions, workspace scope, font custom baseline

[GCWing]

Summary

• Nav search dialog: When opened, loads persisted session metadata from disk for all open workspaces via \sessionAPI.listSessions, merges with in-memory Flow Chat sessions (deduped), and sorts by recency (\lastActiveAt\ / \updatedAt). With an empty query, the sessions column shows a hint instead of listing every session; search matches session id as well as title. Search input uses a text cursor.
• Workspace scope: \sessionMatchesWorkspace\ still matches by id when both are present; if ids differ or the session id is stale, it falls back to path + remote scope so sessions stay tied to the correct nav row.
• Font preference: Choosing the custom size level preserves the currently effective UI base px (preset or valid custom input) instead of resetting to 14.

Testing

• \pnpm run type-check:web\ (passed).