Add share target, prompt templates, command palette and session organisation

- PWA share target: shared text/links start a new chat or go into a
  recent session (chooser dialog)
- Prompt templates (personal + global for admins) with {{placeholders}}
  that are asked for on insert; managed under Settings → Templates
- Ctrl/Cmd+K command palette; Alt+N new session, Alt+Up/Down switch
  sessions, Ctrl+, settings
- Sessions can be pinned, archived and filed into folders; the list can
  be grouped by date or by project, groups are collapsible
- Fix: the "What's new" dialog was never shown to signed-in users

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/types.ts

@@ -64,6 +64,9 @@ export interface Session {
   worktree?: { repo: string; path: string; branch: string; base: string; sub?: string } | null;
   /** Listed because it was shared with the current user. */
   shared?: boolean;
+  pinned?: boolean;
+  archived?: boolean;
+  folder?: string | null;
 }
 
 export interface QuestionOption {