refactor(ui): focus the product on image generation only

Drop the disabled video/3D model tabs from the composer and home model
picker, reword storyboard/campaign copy away from motion/video toward
static campaign frames, ease the home prompt editor font sizing, and
update the deterministic fallback message to describe image generation
and editing instead of "video not yet supported".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/styles.css

@@ -139,8 +139,8 @@ button:disabled {
 
 .prompt-rich-editor.home-prompt-editor {
   color: #1f2937;
-  font-size: 24px;
-  line-height: 1.45;
+  font-size: 20px;
+  line-height: 1.55;
   font-weight: 600;
 }