fix(v0.4): full-screen per device — latest-writer-wins PTY sizing

A shared PTY can only be one size; min-sizing clamped the shared session to the
SMALLEST viewer, so a wide desktop got letterboxed when an iPad was also attached
(iPad looked full-screen, desktop did not).

Switch to latest-writer-wins: the device that most recently fit/focused drives the
PTY size, so whichever device you're actively using is full-screen.
- session.ts: setClientDims resizes the PTY directly (drop min across clients);
  attach/detach/blur never resize (the active device keeps its size)
- frontend: TerminalSession.refit() force-resends dims; window 'focus' /
  visibilitychange re-assert the active tab's size, so switching devices reclaims
  full-screen
- tests updated for latest-wins (incl. join/hidden/blur don't resize)

Verified (two ws clients): 200x50 → iPad 100x40 → desktop refit 200x50 → iPad blur
keeps 200x50. 225 tests green.

public/main.ts

@@ -45,6 +45,14 @@ app.applySettings(settings)
 // Key bar (mobile + desktop) sends to whichever tab is active.
 mountKeybar((data) => app.sendToActive(data))
 
+// Multi-device: when this device regains focus, re-assert the active tab's size
+// so a shared session snaps to THIS screen (latest-writer-wins) — full-screen on
+// whichever device you're currently using.
+window.addEventListener('focus', () => app.refitActive())
+document.addEventListener('visibilitychange', () => {
+  if (!document.hidden) app.refitActive()
+})
+
 // Toolbar utilities.
 mountSearch(toolbar, {
   find: (query, dir) => app.findInActive(query, dir),