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>

core/src/types.rs

@@ -133,6 +133,15 @@ pub struct Session {
     /// Isolated git worktree of this session: {repo, path, branch, base}.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub worktree: Option<Value>,
+    /// Shown at the top of the session list.
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub pinned: bool,
+    /// Hidden from the session list (still searchable).
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub archived: bool,
+    /// User-defined group in the session list.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub folder: Option<String>,
 }
 
 impl Session {