fix(web): complete browser right-panel parity

[ronak-guliani]

Summary

Completes browser/right-panel parity with server-authorized workspace resources, Sidebar localhost previews, and focused integration coverage.

• Workspace reads, assets, and search resolve from tagged thread/project scopes to the authoritative worktree or workspace root; callers cannot enumerate arbitrary filesystem roots.
• Scoped, expiring workspace asset URLs preserve HTML/PDF bytes and MIME types.
• Right-panel browser, terminal, files, HTML/PDF, global commands, responsive sheet/maximize, and Sidebar discovered-port entry points are wired and covered.
• Browser panels preserve host/session state while hidden and clean up all resources when closed.

Security

projects.readFile and projects.searchEntries resolve server-owned thread/project roots. Unknown threads, traversal, absolute paths, adjacent-root forgery, and symlink escapes are rejected and regression-tested.

Coverage

• RightPanelTabs titles, favicons, add menu, context actions, maximize, and middle-click
• Six ChatView surface branches, terminal rendering/switching, global command dispatch, bulk cleanup, responsive sheet/maximize, and browser host/session survival
• Files search/selection/browser promotion and ChatMarkdown HTML/PDF entry points
• Sidebar discovered localhost indicator, thread navigation/focus, preview opening, and surfaced failures

Validation

• pnpm fmt:check
• pnpm lint
• pnpm typecheck
• Focused server authorization and browser acceptance suites
• pnpm test

Head: 0e5d70ada3835b010012a76e14b8b1b98afaafea