feat(sessions): masquer les sessions externes + terminal plein écran

Masquage persistant des sessions Claude découvertes (lancées hors Arboretum)
qui polluent la liste — calqué sur repos.hidden :
- table hidden_sessions (#9) clé claudeSessionId ; helpers DB
- DiscoveryService pose le champ additif SessionSummary.hidden (resume/fork
  restent possibles, survit à un re-scan)
- routes POST/DELETE /sessions/:id/hide, POST /sessions/hide-discovered
  (masse, un clic), GET /sessions?includeHidden=true
- UI : bouton Masquer par ligne, « Masquer l'historique externe », toggle
  « Afficher les masquées », badge masquée ; store + i18n EN/FR + audit

Terminal plein écran (corrige le sizing) :
- SessionView root s'étire (flex-1) → xterm calcule les bonnes dimensions
- refit robuste (requestAnimationFrame + après resync), h-full sur le wrapper
- bouton plein écran (API Fullscreen), tabbar mobile masquée en fullbleed

DialogPrompt : options en liste verticale pleine largeur (plus de troncature
ni de chevauchement sur les libellés longs / multi-lignes).

Doc : centre d'aide (sessions), README EN/FR, site (showcase), CLAUDE.md.
Tests : discovery (hidden) + route hide/unhide/hide-discovered.

README.md

@@ -28,8 +28,8 @@ A single Node.js daemon you run on your dev machine (`npx @johanleroy/git-arbore
 
 - **Worktree-first, multi-repo dashboard** — every worktree of every registered repo, with its git state (branch, ahead/behind, dirty files) *and* the state of its Claude Code session (busy / waiting for input / idle / resumable).
 - **Full worktree lifecycle** — create (with per-repo post-create hooks: `npm ci`, copy `.env`…), adopt worktrees created by hand, delete with guardrails, prune orphans.
-- **Session discovery & resume** — sessions you launched in your own terminal show up automatically; resume dead ones, observe or fork live ones. Never corrupts a live session.
-- **Web terminal** — full xterm.js terminal to every managed session, surviving browser disconnects.
+- **Session discovery & resume** — sessions you launched in your own terminal show up automatically; resume dead ones, observe or fork live ones. Never corrupts a live session. Hide the old ones that clutter the list (one click clears the whole external history; they stay resumable).
+- **Web terminal** — full xterm.js terminal to every managed session, surviving browser disconnects; truly fullscreen, with the prompt pinned to the bottom and full scrollback above.
 - **Supervision from your phone** — installable PWA with push notifications when a session needs you; answer a prompt (its options, or deny) without opening a terminal.
 - **Work groups** — bundle related repos (e.g. an API, its web frontend and its docs) into a named group, then launch **one Claude session that spans all of them at once** (via the CLI's `--add-dir`): a single conversation with one shared context working across every repo, plus a unified view of all their worktrees and a side-by-side multi-terminal grid. Group sessions can either create the same branch worktree in each repo first, or run straight on the main checkouts.