fix(sidebar): drag-release bounce, session-delete terminal cleanup, silent resize

- Sidebar.tsx: useLayoutEffect for the layout-push variables so cleanup+setup
  land in the same pre-paint frame (no full-width flash on drag release);
  applyDrag writes the width only while the panel is open (a closed right
  panel no longer squeezes the conversation when dragging the bottom height);
  flush the final drag frame before stopDragScheduling on all three handles;
  sync centerRect.right on width/corner release so the bottom panel does not
  jump.
- index.ts/pty-manager.ts: close a session's UI terminals immediately on
  session/disposed (they used to linger until reconnectGraceMs expired).
- sidebar.module.css: no highlight on the resize handles while dragging
  (local preference; the corner hover hint stays).

src/pty-manager.ts

@@ -184,6 +184,12 @@ export class PtyManager {
     }
   }
 
+  /** Close every terminal of one session (the conversation was deleted —
+   *  terminals must follow the session, not linger until the grace expires). */
+  closeSession(sessionId: string): void {
+    for (const key of this.keysOf(sessionId)) this.close(key)
+  }
+
   /** Close every terminal (plugin teardown). */
   disposeAll(): void {
     for (const timer of this.pendingCloses.values()) clearTimeout(timer)