feat(desktop): add headless terminal for backend PTY state persistence

[oxwen11]

Summary

Add backend PTY state persistence using @xterm/headless and @xterm/addon-serialize. This enables seamless terminal state restoration during tab switching.

Changes

New Files

• apps/desktop/src/main/terminal/headless-terminal.ts — HeadlessTerminal class with mode tracking (DECSET/DECRST), OSC-7 CWD parsing, and snapshot serialization
• apps/desktop/src/main/terminal/types.ts — TerminalModes and TerminalSnapshot type definitions
• docs/plans/2026-02-05-feat-backend-pty-persistence-plan.md
• docs/plans/2026-02-05-feat-rust-pty-daemon-plan.md

Modified Files

• apps/desktop/src/main/terminal/terminal-manager.ts — Integrate HeadlessTerminal into TerminalInstance, mirror PTY output to headless for state tracking
• apps/desktop/src/shared/contract/terminal.ts — Add TerminalSnapshotSchema and terminal.snapshot RPC endpoint
• apps/desktop/src/main/ipc/router/terminal.ts — Add snapshot handler
• apps/desktop/src/main/terminal/index.ts — Export new types
• apps/desktop/package.json — Add @xterm/headless, @xterm/addon-serialize deps

How It Works

PTY Output → HeadlessTerminal.write() → [state captured]
                    ↓
            DirectPublisher.write() → Stream to Frontend

Tab Switch → terminal.snapshot({ terminalId })
          → HeadlessTerminal.getSnapshot()
          → { snapshotAnsi, rehydrateSequences, modes, cwd }

Note

typecheck:web failures are pre-existing on main (store naming mismatch: currentTaskId vs selectedTaskId), unrelated to this PR.