fix: render the open-with-AI dropdown as an ARIA menu

[EricAndrechek]

Closes #11

The dropdown was a <details> with a plain <ul> inside <main>, so content passes treated the labels as prose (astro-trademarks put a ™ on "Claude").

• Trigger is now a <button> with aria-haspopup="menu", aria-expanded, aria-controls.
• <ul role="menu" aria-labelledby> with <li role="none"> / <a role="menuitem" tabindex="-1">; divider is role="separator".
• Keys: Enter/Space/ArrowDown open on first item, ArrowUp on last; in the menu arrows wrap, Home/End, type-ahead, Escape closes and refocuses the button, Tab closes. Click outside closes. Logic is in src/lib/menu-keys.ts. Re-inits on astro:page-load.
• Behaviour change: activating an item now also closes the menu. No data-tm-skip added (roles alone verified sufficient, see below).
• Tests: pnpm test (node:test, 9 tests), now part of verify and CI.

Verification: ran astro-trademarks transformHtml (registry Claude, contentRoots main, default + Starlight skips) on the markup: roles present -> no mark; roles removed (and hidden removed) -> Claude marked. Keys driven in headless Chromium over CDP against the compiled component script: all pass.

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🤖 Generated with Claude Code

https://claude.ai/code/session_01GgSsyTuDULnYbCwrjB3E51

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