refactor: apply code quality suggestions from task-6-mobile-fab review

Three suggestions from the code review applied:

1. Extract _createSessionInput() factory to eliminate five-line
   duplication between showNewSessionInput and showFabSessionInput.
   Both functions now call the shared factory. Updated 4 existing
   tests to check _createSessionInput's body; added 7 new tests
   covering factory behavior and factory usage in each consumer.

2. Fix .new-session-fab:focus-visible outline color from var(--accent)
   to var(--bg) — the FAB background is already var(--accent), so
   using the same color for the outline created zero contrast.
   Added 1 new test to guard this going forward.

3. Add early-return guard in showFabSessionInput against duplicate
   overlays: if (document.querySelector('.fab-input-overlay')) return;
   Prevents a second overlay being created if called programmatically
   while one is already open. Added 1 new test.

532 tests pass (9 new tests added, 523 unchanged).

muxplex/frontend/style.css

@@ -1234,7 +1234,7 @@ body {
 }
 
 .new-session-fab:focus-visible {
-  outline: 2px solid var(--accent);
+  outline: 2px solid var(--bg);
   outline-offset: 2px;
 }