feat: single cycling height button, panel mutual exclusion, consistent shadows

- Replace 3 S/M/L height buttons with one cycling button (like font size)
- Fix closeAllPanels() to include ctx-panel so Context and Settings menus
  cannot be open simultaneously
- Fix ctxOpenBtn handler to use the same toggle-via-closeAllPanels pattern
  as the settings button
- Align .hdr-dropdown shadow to var(--shadow) instead of hardcoded rgba
- Align #ctx-panel z-index to 200 (match .hdr-dropdown)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/style.css

@@ -235,7 +235,7 @@
             background: var(--surface);
             border: 1px solid var(--border);
             border-radius: 8px;
-            box-shadow: 0 4px 16px rgba(0,0,0,0.4);
+            box-shadow: 0 8px 24px var(--shadow);
             z-index: 200;
             overflow: hidden;
         }
@@ -1168,7 +1168,7 @@
             background: var(--surface);
             border: 1px solid var(--border);
             border-radius: 8px;
-            z-index: 100;
+            z-index: 200;
             box-shadow: 0 8px 24px var(--shadow);
             overflow: hidden;
         }