feat(terminal): add fullscreen mode for terminal sessions

[tab11pm]

Summary

Adds fullscreen mode for terminal sessions. A session can be expanded to fill the entire viewport via a header button or Escape key to exit.

Changes

• App.tsx — tracks fullscreenSessionId state, toggleSessionFullscreen callback
• TerminalCard.tsx — fullscreen prop, exit-fullscreen button, Escape key handler (bubble phase)
• WorkspaceCanvas.tsx — splits rendering into normal layer and fullscreen overlay layer (outside camera transform)
• app.css — .terminal-card--fullscreen overrides (position, dimensions, z-index), .workspace__fullscreen-layer container
• i18n.ts — enterFullscreen / exitFullscreen strings (en/ru)
• package-lock.json — cleanup of stale esbuild optional platform binaries

Design Decisions

1. CSS-only, no IPC bounds changes — fullscreen is purely visual. Session bounds are never sent to the main process, so crash/restart always restores the original layout.
2. Fullscreen layer outside camera transform — the overlay renders in a separate workspace__fullscreen-layer div at z-index: 9999, avoiding conflicts with pan/zoom.
3. Single-session constraint — entering fullscreen for session B auto-exits session A.
4. Escape key in bubble phase — avoids stealing Escape from modals, command palette, or dropdowns.

Documentation

• New doc: docs/fullscreen-mode.md — usage, behavior, architecture overview, i18n reference.

Verification

• Click maximize button on a terminal card → session fills viewport
• Press Escape → exits fullscreen, session returns to original position
• Open fullscreen for session A, then open fullscreen for session B → A exits, B enters
• Close app during fullscreen → restart restores session at original position/size
• Canvas pan/zoom works normally when no session is fullscreen