feat(chat): /btw — side questions in a temporary forked session (#2796)

* feat(chat): /btw command — side questions in a temporary forked session

/btw <question> forks the current session (full context inheritance) and
opens a compact peek panel docked above the composer. The composer itself
becomes the btw input while the panel is open: sends route to the fork,
the placeholder and a mode chip reflect the target, and the stop button
aborts the fork's turn. Closing the panel (or the chip's ✕) destroys the
fork, leaving the main conversation untouched.

The panel shows only the fork's own tail (messages at/after the fork
creation time) and live permission/question cards scoped to the fork.

- chat/btw/BtwPanel: peek sheet (desktop + mobile), fork-tail view,
  auto-close on disappearance, Esc to close
- lib/btw: startBtwSession (fork + rename + routed send), closeBtwPanel
  (close = destroy), filterBtwTailMessages
- ChatInput: btw-mode send routing via SendMessageOptions.sessionId,
  btw-aware activity (stop/abort), placeholder + mode chip
- useSessionActivity: exported for per-session activity reads
- i18n: btw keys across all 11 locales

* fix(chat): keep btw sends isolated

* refactor(chat): rework /btw into a metadata-scoped peek panel

- Link the active btw fork through the parent session's metadata
  (openchamber.btwSessionID) so the panel exists only in the session that
  invoked /btw, follows parent navigation, and survives reloads; the fork
  carries a kind:'btw' marker with its originalSessionID.
- Replace the wall-clock history boundary with the id of the newest cloned
  message (server-generated ascending ids), stored in fork metadata.
- Derive panel identity in useBtwPanelState; useBtwStore shrinks to
  transient per-parent UI state (collapsed/creating/destroying).
- Panel UX: dropdown-style glass surface, chat ScrollShadow, single
  title+chevron collapse toggle, muted header controls, promote action
  (keep as a full session and navigate to it), Esc collapses instead of
  destroying, reserved Working indicator row, streaming auto-follow via
  ResizeObserver keyed on content readiness.
- Add a 'peek' chat surface mode that suppresses per-message controls and
  turn footers inside the panel; user bubbles keep a small gap below.
- Hide btw forks from the sidebar, session switcher, and command palette
  until promoted; mark the fork before inserting it into local stores.
- Delete/archive lifecycle: removing the fork unlinks the parent; removing
  the parent also removes its temporary fork.
- patchSessionMetadata now mirrors updated sessions into live stores.
- Localize new strings across all 12 dictionaries; add unit tests for
  metadata helpers, the btw flow, and the UI store.

* fix(chat): clamp the btw panel below the app header when the keyboard is open

Reuse useMobileAutocompleteMaxHeight (the composer autocomplete precedent)
on the panel's scroll body, reserving the panel header and bottom spacer
height, so the sheet adapts to the visual viewport instead of riding under
the app header on mobile.

* fix(lint): drop unused destructured bindings in sessionBtwMetadata

CI eslint has no underscore ignore pattern; strip metadata keys with typed
copies and delete instead of discard-destructuring.

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/lib/i18n/messages/fr.ts

@@ -1855,6 +1855,18 @@ export const dict = {
   'chat.container.returnToParent.titleNamed': 'Retourner à : {title}',
   'chat.container.returnToParent.title': 'Retour à la session parents',
   'chat.container.returnToParent.label': 'Mère',
+  'chat.btw.destroyAria': 'Détruire cette session btw',
+  'chat.btw.titleFallback': 'session btw',
+  'chat.btw.mainComposerPlaceholder': 'Poser une question dans cette session btw…',
+  'chat.btw.loading': 'Démarrage de la session btw…',
+  'chat.btw.toast.emptyArgument': 'Saisissez une question après /btw',
+  'chat.btw.toast.createFailed': 'Échec du démarrage de la session btw',
+  'chat.btw.toast.destroyFailed': 'Échec de la suppression de la session btw. Elle restera dans la barre latérale.',
+  'chat.btw.working': 'En cours…',
+  'chat.btw.collapseAria': 'Réduire le panneau btw',
+  'chat.btw.expandAria': 'Développer le panneau btw',
+  'chat.btw.promoteAria': 'Conserver comme session à part',
+  'chat.btw.toast.promoteFailed': 'Échec de la conservation de la session btw',
   'chat.container.readOnlySubagentPromptBanner': 'Les sessions de sous-agent ne peuvent pas être invitées.',
   'chat.container.sessionLoadError.title': 'Impossible de charger la session',
   'chat.container.sessionLoadError.description': 'Vérifiez la connexion et essayez de charger à nouveau cette session.',
@@ -3021,6 +3033,7 @@ export const dict = {
   'chat.commandAutocomplete.command.catchUpDescription': 'Rétablir le contexte : ce que vous faisiez et où reprendre.',
   'chat.commandAutocomplete.command.debugDescription': 'Investigation guidée de la cause racine d’un bug avant de proposer une correction.',
   'chat.commandAutocomplete.command.weighDescription': 'Comparer 2 à 3 approches avec compromis et recommandation avant de vous engager.',
+  'chat.commandAutocomplete.command.btwDescription': 'Posez une question annexe dans une session enfant temporaire sans interrompre cette conversation.',
   'chat.commandAutocomplete.command.exploreDescription': 'Vous orienter dans ce codebase : tour d’ensemble de l’architecture et des parties principales.',
   'chat.questionCard.submitFailed': 'Impossible d’envoyer la réponse',
   'chat.questionCard.dismissFailed': 'Impossible d’ignorer la question',