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/client/sidebar.module.css

@@ -132,7 +132,7 @@
 }
 
 .panelResizeActive {
-  background: var(--dsw-alias-interactive-bg-hover-accent);
+  /* Drag feedback disabled by local preference: no highlight while resizing. */
 }
 
 .panelBody {
@@ -195,7 +195,7 @@
 }
 
 .bottomResizeActive {
-  background: var(--dsw-alias-interactive-bg-hover-accent);
+  /* Drag feedback disabled by local preference: no highlight while resizing. */
 }
 
 /* The bottom panel's close control: the app's icon-button pattern (28px
@@ -274,8 +274,8 @@
   touch-action: none;
 }
 
-.cornerHandle:hover,
-.cornerHandle[data-dragging] {
+.cornerHandle:hover {
+  /* Only hover feedback stays; dragging highlight disabled by local preference. */
   background: var(--dsw-alias-interactive-bg-hover-accent);
 }