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/index.html

@@ -110,9 +110,7 @@
                 <div class="ctx-row">
                     <button id="font-size-btn" class="ctx-btn" title="Cycle font size: Normal → Large → Small">Aa</button>
                     <button id="theme-btn" class="ctx-btn" title="Toggle light / dark theme">☾</button>
-                    <button class="ctx-btn" data-height="120" title="Compact — input grows up to ~4 lines">S</button>
-                    <button class="ctx-btn" data-height="240" title="Medium — input grows up to ~8 lines">M</button>
-                    <button class="ctx-btn" data-height="480" title="Large — input grows up to ~16 lines">L</button>
+                    <button id="height-cycle-btn" class="ctx-btn" title="Input size: Compact — click to cycle">S</button>
                     <button id="enter-toggle" class="ctx-btn" title="Toggle send shortcut: Ctrl+Enter ↔ Enter">⌃↵</button>
                 </div>
             </div>