Move the floating theme toggle into the navigation bar

[brylie]

The theme toggle floats over the top-right of the header and sticky navigation. On small screens it can cover content or a focused element (WCAG 2.2 criteria 2.4.11 and 2.5.8). Move it into the navigation bar so it no longer overlaps anything.

Grounding: docs/accessibility-audit-2026-09.md, "Remaining recommendations (not fixed in code)", item 4

Acceptance criteria:

• The toggle sits inside the navigation bar, at desktop and mobile widths
• It never covers focused content when tabbing through the page
• Its target is at least 24 by 24 CSS pixels
• It still works with keyboard and screen reader, and keeps the saved preference
• Checked at 320px and 1280px, in light and dark themes