feat: implement platform-specific expand chat input with mobile optimizations

- Add platform detection (Electron vs Web) for expand behavior
  - Electron: 3-state (normal → 50% → 80%) with double-click
  - Web/Mobile: 2-state (normal → expanded) with instant single tap
- Implement fixed 15-line height for web/mobile (360px, capped)
- Add orientation-aware height calculation (landscape vs portrait)
- Remove tooltip on web/mobile, keep for Electron desktop
- Add responsive placeholder text to prevent overlap on mobile
  - Desktop: "Type your message, @file, @agent, or paste images and text..."
  - Mobile (≤640px): "Type message, @file, @agent..."
- Delete dev-docs/expand-chat-input.md per upstream feedback

Addresses PR feedback to simplify from 3-state to 2-state for web/mobile
while maintaining rich desktop experience in Electron app.

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

@@ -163,11 +163,17 @@
   backdrop-filter: blur(8px);
 }
 
-.prompt-expand-button:hover::after {
+/* Only show tooltip on hover for Electron (desktop-mode) */
+.prompt-expand-button.desktop-mode:hover::after {
   opacity: 1;
   transform: translateY(0);
 }
 
+/* Web/Mobile: No tooltip (data-tooltip will be undefined anyway) */
+.prompt-expand-button.web-mode::after {
+  display: none;
+}
+
 .prompt-overlay-text {
   display: inline-flex;
   align-items: center;