feat(desktop): unify dblclick-maximize across mac/win/linux

[attson]

Summary

• After PR feat(desktop): unified title bar across mac/win/linux #64 (unified title bar), double-clicking the empty title bar area did nothing on macOS — TitleBarHiddenInset lets the WebKit view eat the dblclick before NSWindow can route it to system zoom.
• Drop the darwin-specific guard in onTitleDblClick so all three platforms run the same WindowToggleMaximise() path.
• Test reversal: the prior "darwin does NOT call WindowToggleMaximise" assertion is now "darwin DOES call WindowToggleMaximise". 16 tests in TitleBar.test.ts, count unchanged.

Test plan

• macOS: double-click empty area of title bar → window maximizes; double-click again → restores.
• Windows / Linux: behavior unchanged from PR feat(desktop): unified title bar across mac/win/linux #64.
• npm test from desktop/frontend/: 363/363 pass.

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