feat(desktop): strict per-thread drafts on decoupled composer

Keyed draft stash (Map + localStorage mirror) behind the live composer:
switching threads stashes the departing draft and restores the entering
one; empty threads show an empty box. Session lifecycle never clears
composer state — the scope swap is the only coupling.

Co-authored-by: mollusk <roger@roger.local>
Co-authored-by: Teknium <127238744+teknium1@users.noreply.github.com>

apps/desktop/src/app/session/hooks/use-session-actions.ts

@@ -328,8 +328,7 @@ export function useSessionActions({
       setYoloActive(false)
       setCurrentCwd(workspaceCwdForNewSession())
       setCurrentBranch('')
-      // Never clear the composer here: it sits above the thread and its
-      // contents (text + attachments) follow the user across session changes.
+      // Never clear the composer here — ChatBar's per-thread draft swap owns it.
       setFreshDraftReady(true)
     },
     [activeSessionIdRef, busyRef, navigate, selectedStoredSessionIdRef]