feat: dock mobile keybar to bottom of screen with larger touch targets

The mobile keybar was a small floating overlay that obscured terminal
content. Now it's a full-width bar docked to the bottom using flexbox
layout, so the terminal shrinks to accommodate it. Buttons use 44px
height (Apple HIG recommended touch target) and evenly fill the width.
Also uses 100dvh for correct mobile viewport sizing.

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

bun.lock

@@ -13,7 +13,7 @@
     },
   },
   "packages": {
-    "ghostty-web": ["ghostty-web@github:rcarmo/ghostty-web#fcc47d4", {}, "rcarmo-ghostty-web-fcc47d4"],
+    "ghostty-web": ["ghostty-web@github:rcarmo/ghostty-web#fcc47d4", {}, "rcarmo-ghostty-web-fcc47d4", "sha512-tq0cFciI32VTyOXDoLHQQDndeA6jhFuZ/3TWYx3VlYDzRhYkWAtTBi6t29isYPzdiKNIWggjkn3Ve/+Qub/wBg=="],
 
     "typescript": ["typescript@5.9.3", "", { "bin": { "tsc": "bin/tsc", "tsserver": "bin/tsserver" } }, "sha512-jl1vZzPDinLr9eUt3J/t7V6FgNEw9QjvBPdysz9KfQDD41fQrC2Y4vKQdiaUpFT4bXlb1RHhLpp8wtm6M5TgSw=="],
   }