feat(home): changelog modal, template focus fix, in-app release notes

[srfwb]

Summary

Two user-reported issues plus a new feature from the April 26 polish session.

What changed

User-facing

• Clicking a template button in the "Nouveau projet" modal no longer leaves a parasitic amber outline after releasing the mouse. The selection indicator (amber border + tinted background) now appears immediately on the active template, not delayed until after hover ends.
• "Nouveautés de la vX.Y.Z" in the Home rail footer is now a live button. Clicking it opens a modal displaying the full in-app changelog — written in French, version by version, from the current release back to v0.1.0.
• The version label on the link updates automatically from package.json (same APP_VERSION constant used by the status bar and the Home brand).

Internal

• src/styles/global.css — .modal__template:focus-visible { outline: none; border-color: var(--accent) } suppresses the generic focus ring on template buttons. The --active modifier already handles the visual selection state.
• src/changelog.md — new user-facing changelog in French, following a simplified Keep a Changelog structure (Ajouté / Amélioré / Corrigé). Imported at build time via Vite's ?raw suffix.
• src/home/sections/ChangelogModal.tsx — modal component that renders the Markdown with a built-in minimal parser (headings, bold, lists, rules). Uses useModalA11y for Escape / focus trap / focus return.
• src/home/rail/HomeRailFoot.tsx — "Nouveautés" link is now a <button> that toggles the changelog modal. Version label reads from APP_VERSION.
• src/styles/global.css — new .cl-* class family for the changelog modal body (headings, list items, horizontal rules).
• Convention added to project CLAUDE.md: update src/changelog.md (French, in-app) alongside CHANGELOG.md (English, GitHub) on every release.