fix(audit): bootstrap crash, autosave toast, rename validation, tab cleanup, visual polish

[srfwb]

Summary

Batch of fixes from the ultra-review audit. Addresses 5 bugs and 4 visual issues identified during the April 25 testing session.

What changed

User-facing

• App no longer crashes on startup if the last-opened project's folder was deleted externally — falls back to the Home view with a console warning.
• Disk autosave failures now show a red toast ("Impossible de sauvegarder /path sur le disque") instead of silently losing edits.
• Creating a project when a folder with the same name already exists shows a clean French message instead of leaking a raw path.
• Renaming a project to a name already used by another project is now rejected with an error.
• Switching projects clears the previous project's open tabs so ghost tabs don't persist.
• Project card stagger animation is snappier (25 ms step, 120 ms cap — was 40 ms / 240 ms).
• Editor tabs show a visible amber underline on keyboard focus.
• Home rail nav items are now proper <button> elements with hover and focus states instead of inert <div>s.
• Rail footer links ("Docs et aide", "Nouveautés") show a pointer cursor on hover.

Internal

• src/projects/bootstrap.ts — try/catch around openProject in bootstrap.
• src/projects/diskAutoSave.ts — toast.error() on write failure.
• src/projects/actions.ts — friendlier error on duplicate folder, duplicate-name check in renameProject, setState({ openFiles: [], activeFile: null }) before setView("ide") on project switch.
• src/styles/global.css — .tab:focus-visible with amber underline, .home-nav-item hover/focus/disabled states, .home-rail-link cursor pointer, reduced proj-in stagger.
• src/home/rail/HomeNavItem.tsx — <div> → <button> with optional onClick and disabled.