feat(desktop): unified title bar across mac/win/linux

[attson]

Summary

• Merge top toolbar with native title bar on all three platforms — IDE-style single row.
• macOS: Mac.TitleBar = TitleBarHiddenInset() puts traffic lights over the merged bar; Windows + Linux switch to Frameless: true and self-draw min/max/close.
• New components: TitleBar.vue (sole owner of merged bar, detects platform via Wails Environment()), WindowControls.vue (Win/Linux chrome buttons), useWindowMaximized composable (shared maximize state).
• 8 px .app inset on Win/Linux to compensate for borderless-maximized overflow; gated off on macOS where the window is not frameless.
• Go side: main.go split into per-OS build-tagged platformOptions() merged via mergePlatformOptions().
• Drops the redundant "AT Term" brand text — already shown by OS title.
• Bundles an unrelated fix for QuickInput Enter dispatch (separate \r send so Codex doesn't see it as pasted multiline input).

Test plan

• macOS: traffic lights overlay merged bar, no separate native title above. Drag from empty area moves window; clicking buttons doesn't drag.
• macOS: double-click empty area triggers system zoom; 8 px inset is NOT applied when zoomed.
• Windows: three buttons render and function (min, max/restore, close). Maximized window doesn't overflow edges. Double-click title bar toggles maximize; snap-to-edge works.
• Windows: close button with active sessions opens confirm-quit dialog.
• Linux (X11 — GNOME/KDE): three buttons functional, drag region works. Edge-resize known limitation (use maximize button or WM shortcuts).
• npm test from desktop/frontend/: 363/363 pass.
• go test ./desktop/... passes.
• go build ./desktop/... succeeds.

Spec: docs/superpowers/specs/2026-05-20-unified-titlebar-design.md
Plan: docs/superpowers/plans/2026-05-20-unified-titlebar.md