fix(split-pane): address the rest of Ark0N's PR #453 review

- Exclude popped-out (detached) sessions from the split picker:
  SplitTerminalPane._sendResize() has no yield-to-detached-window check
  the way the primary pane's sendResize() does, so splitting against a
  detached session put its own window and Pane B in a fight over the
  same PTY's dimensions. Simplest fix per the review: keep them out of
  buildSplitPickerSessions() entirely.
- Show a visible dead state when Pane B's WebSocket drops. onData
  already silently discards keystrokes while the socket isn't OPEN
  (there is no reconnect for v1), so a dropped socket left the pane
  looking normal while it quietly ate everything typed into it.
- openSplitPane() returns early with no active session, so a split
  triggered from the home screen no longer creates and connects Pane B
  behind the opaque welcome overlay with nothing to show for it.
- onMove() during a divider drag now bails when the split has
  auto-collapsed mid-drag (the other pane's session ending) instead of
  throwing on `divider.parentElement` being null.
- Promote Pane B via `selectSession(id, { auto: true })` when Pane A's
  session ends — this is an app-driven selection, not the user clicking
  a tab, so it must not spend the promoted session's idle alert.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/split-pane-auto-collapse-unit.test.ts

@@ -83,7 +83,9 @@ describe('terminal-split.js _onSessionDeleted wrapper (I6)', () => {
     expect(app.closeSplitPane).toHaveBeenCalledTimes(1);
     // Pinned ordering: selectSession must receive the id _splitSessionId held
     // BEFORE closeSplitPane ran (which nulls it), not whatever it holds after.
-    expect(app.selectSession).toHaveBeenCalledWith('session-b');
+    // { auto: true } because this is an app-driven promotion, not the user
+    // clicking a tab — it must not spend the promoted session's idle alert.
+    expect(app.selectSession).toHaveBeenCalledWith('session-b', { auto: true });
     expect(app.__originalDeletedCalls).toEqual([{ id: 'session-a' }]);
   });