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/terminal-session.ts

@@ -349,6 +349,19 @@ export class TerminalSession {
     })
   }
 
+  /**
+   * Re-fit and FORCE-resend dims (latest-writer-wins): when this device regains
+   * focus, re-assert its size so the shared PTY snaps to this screen even if
+   * another device had resized it. No-op while hidden.
+   */
+  refit(): void {
+    if (this.el.style.display === 'none') return
+    this.lastCols = -1 // force sendResize to fire even if our dims are unchanged
+    this.lastRows = -1
+    const dims = this.safefit()
+    if (dims !== null) this.sendResize(dims.cols, dims.rows)
+  }
+
   hide(): void {
     this.el.style.display = 'none'
     // v0.4: withdraw our size vote so a backgrounded mirror doesn't clamp the