Fix responsive header layout after adding Editor

[AlexMikhalev]

Adding “Try the editor” made the fixed-height header wrap and clip at common desktop widths; wide-screen spacing also broke it again at 2,560px. This shortens the navigation label to “Editor”, keeps desktop links on one line with consistent spacing, and uses the existing compact menu below 1,280px. The expanded compact menu scrolls on short screens. All ten destinations, search and the client version badge are preserved.

Adds Chromium, Firefox and WebKit CI checks against the actual built-site artifact: 15 widths on two pages, normal/fallback fonts, no clipping or overlapping controls, exact link origins, keyboard search/Editor/Donate activation and menu resizing. The test build uses a local base URL and checks stylesheet identity so it cannot accidentally test production CSS.

Validation:

• Reproduced the original wrapping and offscreen controls with the browser checks before applying the CSS fix.
• Zola 0.22.1 build and all 25 existing Python tests pass.
• Local Chromium, Firefox and WebKit each passed 60 geometry cases plus keyboard/search/menu checks (180 geometry cases total).
• Native Brave toolbar confirmed actual 200% zoom; header remained readable with every destination and search visible. Zoom restored afterward.
• Independent review found no blocking issues; exact-origin assertions and the macOS-only WebKit keyboard workaround incorporate its feedback.
• Workflow lint passes. All five CI jobs pass, including another 180 geometry cases and interaction checks across Linux Chromium, Firefox and WebKit. Native Safari is not claimed by the WebKit run.

Production deployment remains the existing main-branch workflow. No editor bundle or demo/download bytes change.