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/tabs.ts

@@ -337,6 +337,12 @@ export class TabApp {
     this.tabs[this.activeIndex]?.session.send(data)
   }
 
+  /** Re-assert the active tab's size (latest-writer-wins) when this device
+   *  regains focus, so a shared session snaps back to this screen's size. */
+  refitActive(): void {
+    this.tabs[this.activeIndex]?.session.refit()
+  }
+
   /** Apply theme + font settings to every terminal (M3). */
   applySettings(s: Settings): void {
     this.settings = s