Chat tabs, chat mode, own modes, compaction notice, CLI binaries again

- Terminals and file managers as tabs of a chat; chat terminals keep running on
  the server (no idle timeout) and re-attach from any device
- "Just chat" in New session and a built-in chat mode with a lean prompt and only
  web tools; custom modes with kind (coding/conversation), read-only and tool picker
- Mode selector as a dropdown
- The chat shows running compactions (automatic: tokens and share of the limit)
  and labels summaries
- Docker image builds and serves the terminal client binaries again
- Fix: atomic session updates; background title/usage writes no longer undo each
  other (auto compaction triggered one turn late)

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

server/src/app.rs

@@ -46,6 +46,8 @@ pub struct Runtime {
 }
 
 pub struct App {
+    /// Serializes read-modify-write of sessions (see `update_session`).
+    session_write: Mutex<()>,
     config_path: PathBuf,
     config: RwLock<Arc<Config>>,
     registry: RwLock<Arc<Registry>>,
@@ -101,6 +103,7 @@ impl App {
         let registry = Registry::build(&cfg).await;
         let (events, _) = broadcast::channel(4096);
         let app = Arc::new(Self {
+            session_write: Mutex::new(()),
             config_path,
             config: RwLock::new(Arc::new(cfg)),
             registry: RwLock::new(Arc::new(registry)),
@@ -326,10 +329,27 @@ impl App {
         Ok(s)
     }
 
+    /// Load, change and save a session atomically: background tasks (title, usage, phase,
+    /// todos) change different fields at the same time and must not undo each other.
+    pub fn update_session(&self, id: &str, f: impl FnOnce(&mut Session)) -> Result<Session> {
+        self.modify_session(id, true, f)
+    }
+
+    /// Like `update_session`; `touch = false` keeps `updated_at` (organising the list).
+    pub fn modify_session(&self, id: &str, touch: bool, f: impl FnOnce(&mut Session)) -> Result<Session> {
+        let _guard = self.session_write.lock().unwrap();
+        let mut s = self.session(id)?;
+        f(&mut s);
+        if touch {
+            return self.save_session(s);
+        }
+        self.store.save_session(&s)?;
+        self.emit(Event::SessionUpdated { session: s.clone() });
+        Ok(s)
+    }
+
     pub fn set_todos(&self, session_id: &str, todos: Value) -> Result<()> {
-        let mut s = self.session(session_id)?;
-        s.todos = todos;
-        self.save_session(s)?;
+        self.update_session(session_id, |s| s.todos = todos)?;
         Ok(())
     }