fix: prevent cumulative terminal shrinkage in fullscreen mode

When switching terminal sessions in fullscreen mode, the viewport
shrank cumulatively because .terminal-wrapper uses
grid-template-rows: auto 1fr. With showControls=false, the single
child (.terminal-container) landed in the auto track instead of 1fr,
creating a feedback loop with xterm fit().

- Add .terminal-wrapper.no-controls with grid-template-rows: 1fr
  so the container fills the wrapper when the header is hidden.
- Apply no-controls class in TerminalComponent when showControls=false.
- Replace setTimeout(50) with double requestAnimationFrame in
  TerminalPage for more reliable fit() timing after tab switches.

Quality gates: tsc --noEmit (clean), pytest (208 passed, 6 pre-existing)

openspec/changes/terminal-fullscreen-unified-header/.openspec.yaml

@@ -1,6 +1,6 @@
 name: terminal-fullscreen-unified-header
-status: in-progress
-phase: apply
+status: completed
+phase: verify
 type: bugfix
 description: Fix terminal fullscreen mode where session switcher overlays terminal controls. Combine both elements into a single unified header visible at all times.
 created_at: 2026-05-28