sidebar: + menu always reachable on empty panes, unified default width (content-area half), expand/collapse animation

- An empty pane (fresh split, or last tab dragged out) now keeps the tab
  strip with its + menu mounted, so a new tab can always be created or
  dragged in; the placeholder body still explains the state.
- Default width unified: new sessions open at half the CONTENT area
  (viewport minus the left sidebar), so the panel never swallows half the
  whole window and old/new sessions no longer disagree (previously old
  persisted widths ~1/4 vs fresh sessions 1/2). Fullscreen restore targets
  the same content-half width.
- Expand/collapse now animates: the panel stays mounted and slides
  off-screen (translateX + visibility after the slide); fullscreen width
  changes animate too, and drag-resizing disables the transition.

src/client/sidebar.module.css

@@ -57,6 +57,27 @@
   background: var(--dsw-alias-bg-layer-1);
   border-left: 1px solid var(--dsw-alias-border-l2);
   box-shadow: var(--dsw-shadow-lv2);
+  /* Expand/collapse slides the panel; fullscreen animates the width. */
+  transition:
+    transform var(--ds-transition-duration-slow) var(--ds-ease-in-out),
+    width var(--ds-transition-duration-slow) var(--ds-ease-in-out);
+}
+
+/* Collapsed: slid off-screen, interactive nothing, hidden after the slide. */
+.panelHidden {
+  transform: translateX(102%);
+  pointer-events: none;
+  box-shadow: none;
+  visibility: hidden;
+  transition:
+    transform var(--ds-transition-duration-slow) var(--ds-ease-in-out),
+    width var(--ds-transition-duration-slow) var(--ds-ease-in-out),
+    visibility 0s linear var(--ds-transition-duration-slow);
+}
+
+/* Width drags write at pointer cadence; easing would detach the edge. */
+.panel[data-dragging] {
+  transition: none;
 }
 
 .panelResize {