Fix composer send button clipped off-screen on mobile (#24)

The composer's message textarea lacked min-width: 0, so on browsers
that compute a larger default intrinsic width for flex-child form
controls (observed on Firefox for Android), the row could overflow
and push the fixed-width send button past the viewport edge instead
of shrinking the textarea to fit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/Composer.css

@@ -15,6 +15,7 @@
 
 .composer-box textarea {
   flex: 1;
+  min-width: 0;
   resize: none;
   background: var(--ds-surface-2);
   border: 1px solid var(--ds-border);