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.

test/session.test.ts

@@ -208,54 +208,51 @@ describe('attachWs', () => {
     expect(received(b)).toEqual([{ type: 'output', data: 'live' }]);
   });
 
-  it('resizes the PTY to the MIN dims across active viewers (tmux-style)', () => {
+  it('attaching alone does NOT change the PTY size (the active device keeps it)', () => {
     const s = newSession(); // spawn dims 80x24
     const wide = createMockWs();
-    const narrow = createMockWs();
-
-    // Attach alone does NOT vote on size (a join could be a hidden mirror).
-    attachWs(s, wide);
-    expect(s.pty.cols).toBe(80);
-
-    // A client only constrains the PTY once it reports dims (it's viewing).
     setClientDims(s, wide, 200, 50);
     expect(s.pty.cols).toBe(200);
+
+    const hidden = createMockWs();
+    attachWs(s, hidden); // joins but never reports dims (background mirror)
+    // The join must not change the PTY — the active viewer keeps 200x50.
+    expect(s.pty.cols).toBe(200);
+    expect(s.pty.rows).toBe(50);
+  });
+
+  it('latest-writer-wins: the most recent client dims drive the PTY size', () => {
+    const s = newSession();
+    const desktop = createMockWs();
+    const ipad = createMockWs();
+
+    setClientDims(s, desktop, 200, 50);
+    expect(s.pty.cols).toBe(200);
     expect(s.pty.rows).toBe(50);
 
-    attachWs(s, narrow);
-    setClientDims(s, narrow, 90, 30);
-    // min(200,90)=90, min(50,30)=30
+    // The device used next (smaller iPad) takes over — it is now full-screen.
+    setClientDims(s, ipad, 100, 40);
+    expect(s.pty.cols).toBe(100);
+    expect(s.pty.rows).toBe(40);
+
+    // Switching back to the desktop (re-fit on focus) reclaims its size.
+    setClientDims(s, desktop, 200, 50);
+    expect(s.pty.cols).toBe(200);
+    expect(s.pty.rows).toBe(50);
+  });
+
+  it('clearClientDims (tab hidden) does NOT resize — the active device keeps its size', () => {
+    const s = newSession();
+    const a = createMockWs();
+    const b = createMockWs();
+    setClientDims(s, a, 200, 50);
+    setClientDims(s, b, 90, 30); // b is using it now → 90x30
+
+    clearClientDims(s, a); // a's tab hidden elsewhere → just forget a's dims
+    // PTY unchanged: b is still the active viewer at 90x30.
     expect(s.pty.cols).toBe(90);
     expect(s.pty.rows).toBe(30);
   });
-
-  it('a hidden mirror (no dims reported) does NOT clamp the shared PTY', () => {
-    const s = newSession();
-    const viewer = createMockWs();
-    const hidden = createMockWs();
-
-    attachWs(s, viewer);
-    setClientDims(s, viewer, 200, 50);
-    attachWs(s, hidden); // joins but never reports dims (background tab)
-
-    // The hidden mirror must not drag the PTY down to the 80x24 default.
-    expect(s.pty.cols).toBe(200);
-    expect(s.pty.rows).toBe(50);
-  });
-
-  it('clearClientDims withdraws a vote (tab hidden) and lets the PTY grow', () => {
-    const s = newSession();
-    const wide = createMockWs();
-    const narrow = createMockWs();
-    attachWs(s, wide);
-    setClientDims(s, wide, 200, 50);
-    attachWs(s, narrow);
-    setClientDims(s, narrow, 90, 30); // clamps to 90x30
-
-    clearClientDims(s, narrow); // narrow's tab hidden → withdraw vote
-    expect(s.pty.cols).toBe(200);
-    expect(s.pty.rows).toBe(50);
-  });
 });
 
 // ── detachWs (remove one client) ──────────────────────────────────────────────
@@ -280,19 +277,19 @@ describe('detachWs', () => {
     expect((s.pty as MockIPty).killed).toBe(false);
   });
 
-  it('lets the PTY grow back when a small client leaves', () => {
+  it('does NOT resize when a client leaves (the remaining device keeps its size)', () => {
     const s = newSession();
     const wide = createMockWs();
     const narrow = createMockWs();
     attachWs(s, wide);
     setClientDims(s, wide, 200, 50);
     attachWs(s, narrow);
-    setClientDims(s, narrow, 90, 30); // clamps to 90x30
+    setClientDims(s, narrow, 90, 30); // narrow is the latest writer → 90x30
 
     detachWs(s, narrow, 5_000);
-    // only the wide client remains → PTY expands back to 200x50
-    expect(s.pty.cols).toBe(200);
-    expect(s.pty.rows).toBe(50);
+    // detach does not resize; PTY stays at the last set size until a client re-fits.
+    expect(s.pty.cols).toBe(90);
+    expect(s.pty.rows).toBe(30);
   });
 
   it('keeps the PTY alive after the last detach: further onData still fills the buffer', () => {