refactor(app): restore terminal-first shell

[chareice]

Summary

• restore the desktop TabBar and host switcher plus the terminal-only mobile workbench
• add a shared on-demand Workspace Manager: desktop drawer and mobile bottom sheet over the same workspace-to-terminal tree
• support workspace create, rename, delete, reorder and terminal select, create, move, and close without restoring session-mode navigation
• remove frontend agent-session navigation, chat/create flows, browser state, API wrappers, and E2E coverage
• preserve compression, previous-tab keepalive, mobile input and touch-scroll fixes, and forget-host support

Product boundary

The browser is terminal-only again. Workspaces remain the organization model, managed through the top tabs for fast switching and the Workspace Manager for complete management. This does not delete backend ACP data or routes; it removes the unfinished mixed session UX while keeping server and storage compatibility dormant.

Validation

• pnpm typecheck
• pnpm test (38 files, 292 tests)
• pnpm build
• pnpm e2e:test (84 tests, including desktop and mobile Workspace Manager flows)
• git diff --check