fix(polish): widen palette, remove FLIP anim, kill context-menu focus ring, allow open-path

[srfwb]

Summary

Polish batch from the April 25 testing session. Four fixes shipped on a single branch.

What changed

User-facing

• Palette widened from 720 px to 800 px max.
• Palette open animation reverted to the gentle scale+fade (0.96 → 1, −8 px slide, 180 ms). The FLIP animation that measured the search bar's position was removed — it added complexity without improving the feel.
• Right-clicking a context-menu item no longer shows an amber focus ring. The generic :focus-visible rule from A6 was hitting .context-menu__item; an override now shows a subtle background instead.
• "Ouvrir le dossier" in the Continue card and the project-card context menu now works: switched from openPath (which silently fails on directories on Windows) to revealItemInDir (explicitly asks the OS to open the file explorer at that path). Added opener:allow-open-path and opener:allow-reveal-item-in-dir permissions.

Internal

• src/home/sections/HomeSearch.tsx — removed FLIP measurement code (getBoundingClientRect, CSS custom properties), removed useRef.
• src/home/sections/ContinueCard.tsx + ProjectCard.tsx — openPath → revealItemInDir.
• src/styles/global.css — .palette-root width 720px → 800px, animation palette-enter restored to first version, .context-menu__item:focus-visible override added.
• src-tauri/capabilities/default.json — added opener:allow-open-path and opener:allow-reveal-item-in-dir.