fix(nav): restore full-width navbar — remove floating bubble design
The navbar in page view was re-implemented as a centered floating bubble
(position: fixed, centered at 50%, max-width 600px, box-shadow, rounded
corners) which lost the carefully-tuned full-width design.
Restore the original navbar design from commit 2eb7a7037b92:
- position: absolute, top: 0, left: 0, right: 0 (full window width)
- No border, no border-radius, no box-shadow (no bubble effect)
- -webkit-app-region: drag (navbar is the window drag handle)
- 36px height matching the original design
- Trigger zone back to 50px (was shrunk to 12px)
Keep the webview push-down mechanism (.navbar-active class) that was
added later to solve Electron's composited <webview> layer rendering
over host DOM — but adjust push-down from 90px to 36px to match the
navbar height exactly.