feat: implement expandable chat input with double-click detection and gradient tooltip

- Add expand button with Maximize2/Minimize2 icons
- Implement 3-state height management (normal/50%/80%)
- Smart double-click detection with 300ms delay
- Height calculation based on session-view - 200px message space
- Custom CSS tooltip with dark gradient background and backdrop blur
- Send button anchored at bottom via margin-top: auto
- Smooth CSS transitions throughout
- Double-click at 80% now reduces to 50% (not normal)
- Removed all debug console.log statements

packages/ui/src/styles/messaging/prompt-input.css

@@ -13,7 +13,7 @@
 }
 
 .prompt-input-actions {
-  @apply flex flex-col items-center justify-between;
+  @apply flex flex-col items-center;
   align-self: stretch;
   height: 100%;
   padding: 0.5rem 0.25rem;
@@ -122,6 +122,7 @@
   background-color: rgba(15, 23, 42, 0.04);
   transition: background-color 0.15s ease, color 0.15s ease;
   padding: 0;
+  position: relative;
 }
 
 .prompt-expand-button:hover:not(:disabled) {
@@ -140,6 +141,33 @@
   cursor: not-allowed;
 }
 
+.prompt-expand-button::after {
+  content: attr(data-tooltip);
+  position: absolute;
+  top: calc(100% + 8px);
+  right: 0;
+  background: linear-gradient(135deg, rgba(30, 41, 59, 0.95) 0%, rgba(15, 23, 42, 0.98) 100%);
+  color: var(--text-primary);
+  padding: 0.5rem 0.75rem;
+  border-radius: 6px;
+  font-size: 0.75rem;
+  line-height: 1.4;
+  white-space: nowrap;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.25), 0 0 0 1px rgba(255, 255, 255, 0.1);
+  border: 1px solid var(--border-base);
+  pointer-events: none;
+  opacity: 0;
+  transform: translateY(-4px);
+  transition: opacity 0.2s ease, transform 0.2s ease;
+  z-index: 1000;
+  backdrop-filter: blur(8px);
+}
+
+.prompt-expand-button:hover::after {
+  opacity: 1;
+  transform: translateY(0);
+}
+
 .prompt-overlay-text {
   display: inline-flex;
   align-items: center;
@@ -211,6 +239,7 @@
   @apply w-10 h-10 rounded-md border-none cursor-pointer flex items-center justify-center transition-all flex-shrink-0;
   background-color: var(--accent-primary);
   color: var(--text-inverted);
+  margin-top: auto;
 }
 
 .send-button.shell-mode {