fix: fully collapse hidden views — zero out all box model properties

The hidden primary view was creating a gap above the active view
because flex:0 and height:0 alone don't override min-height from
the component's shadow DOM :host styles. In flex layout, the
default min-height:auto can prevent an element from collapsing
to zero height.

Added to .view-hidden:
- min-height: 0 — overrides flex min-height:auto
- max-height: 0 — belt-and-suspenders height constraint
- padding: 0 — prevents padding from creating space
- margin: 0 — prevents margin from creating space
- border: none — prevents border from creating space
- flex: 0 0 0px — explicit flex-basis:0px (not just flex:0)

All with !important to override shadow DOM :host styles.

frontend/index.css

@@ -161,8 +161,13 @@ body div.sidebar {
    display:none discards scroll state in WebKitGTK. */
 .main-panel > .view-hidden {
     visibility: hidden !important;
-    flex: 0 !important;
+    flex: 0 0 0px !important;
+    min-height: 0 !important;
+    max-height: 0 !important;
     height: 0 !important;
+    padding: 0 !important;
+    margin: 0 !important;
+    border: none !important;
     overflow: hidden !important;
     pointer-events: none !important;
     contain: strict !important;