Close mobile layers with the phone's Back

[IvoryHeart]

Summary

On phones, the system Back gesture or button currently leaves Roamgate, even when the tab sheet, the input dock or a mobile view such as Files is open. This PR makes Back close the open layer first, the way native apps behave.

• New hook web/src/systemBack.ts: useSystemBack(open, close, level). It is active only in the mobile layout.
• Wired to three layers:
  • the tab sheet (popup)
  • the input dock (panel, active pane only)
  • any mobile view other than the session, such as Workspaces, Files, Changes or Ranger (panel). Back returns to the session through activateTerminalSurface.
• How it works:
  • While any layer is open, the current history entry is one guard entry at the same URL.
  • Back pops the guard and closes the topmost layer: the higher level first, newest first within a level. The guard returns while layers remain.
  • When the last layer closes with its on-screen control, the guard is removed once, even if several layers closed in the same React commit. The next Back leaves the page in one press.
  • A layer opened while that removal is pending (for example, a tab chosen from the sheet that opens a view) keeps its entry.
  • Unmounting leaves history alone. Desktop is unchanged.
• FEATURES.md: one line under Mobile and PWA.
• scripts/check-web-assets.mjs: the initial JS budget goes from 802 to 804 KiB. main measures 802.0/802 KiB; with the hook it is 803.0 KiB, and gzip is 245.6/256 KiB.

Tests

• web/src/systemBack.test.ts. It uses happy-dom in an isolated runtime, following TerminalView.inputMode.test.ts, and renders real React commits under StrictMode. It covers:
  • layers closed together
  • on-screen closes mixed with system Back
  • two layers registering in one commit across a layout change (stacking must not depend on React effect order)
  • a layer opened while a Back is pending
  • unmount
• TerminalView.inputMode.test.ts: its layoutPreferences mock gains useLayoutPreferences, because TerminalView now calls the hook.

Validation

• bun run format:check, bun run lint, bun run typecheck and bun run typecheck:bun pass. bun run test:quick: 3720 pass, 0 fail.
• The same design ships in a downstream derivative (Herdr World). There it was exercised by headless-Chrome browser tests and on a physical phone.
• Tried by hand on a physical phone against a real Herdr backend: Back closed the tab sheet, the input dock and mobile views as expected. The device and browser weren't recorded.

🤖 Generated with Claude Code

https://claude.ai/code/session_01AWodzpMyqdBcX25EE9HhDG