fix(mobile): overflow breaks — tables, composer, call bar, previews, cards (M1)

Mobile-audit batch 1. All fixes reuse cinny's own responsive primitives and are
mobile-gated so desktop is unchanged.

- Message tables: wrap <table> in an overflow-x container so a wide table scrolls
  instead of overflowing the message column / page body.
- Composer toolbar: let the before|editable|after row and the toolbar wrap on
  phones (@media <=750px) instead of squeezing the editable to zero and pushing
  Send off-screen.
- In-call control bar: collapse to the compact/stacked layout on a mobile
  viewport (ScreenSize.Mobile) too, not just when the bar's own container is
  <500px — fixes the 500-750px band where the control row overflowed.
- URL-preview card: base width toRem(400) -> min(25rem, 92vw) so a single card
  fits a narrow phone (still exactly 400px on desktop).
- Explore card grid: drop to one column at <=750px (was a fixed 3-col grid).

Two review passes: desktop behavior provably unchanged (all gated by @media /
ScreenSize.Mobile; the table wrapper only contains previously-overflowing tables).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/url-preview/UrlPreview.css.tsx

@@ -4,7 +4,9 @@ import { DefaultReset, color, config, toRem } from 'folds';
 export const UrlPreview = style([
   DefaultReset,
   {
-    width: toRem(400),
+    // 25rem (=400px) on desktop, but shrink to fit narrow phones so a single
+    // card doesn't exceed the viewport (mirrors UrlPreviewWide's min()).
+    width: 'min(25rem, 92vw)',
     minHeight: toRem(102),
     backgroundColor: color.SurfaceVariant.Container,
     color: color.SurfaceVariant.OnContainer,