breakglass UI: mobile-first rework (chat input was hidden on phones)

Mobile is the primary client for fixing the devvm, but the first cut was
desktop-first and the chat input was unreachable on a phone:
- Root cause: the shell used height:100%/100vh, so on mobile browsers the
  composer at the bottom sat behind the address/tool bar — you saw the VM
  buttons but no place to type. Switched #app to 100dvh (dynamic viewport
  height) with a 100vh fallback; body no longer scrolls (chat scrolls
  internally), killing iOS rubber-banding.
- Layout is now mobile-first single-column: the chat fills the screen with
  the composer pinned at the bottom and always visible. The VM power controls
  moved into a slide-up bottom sheet behind a compact "⚡ VM" header button
  (backdrop + close + grab handle). At ≥900px the sheet becomes a static side
  column again and the toggle is hidden — desktop unchanged.
- Touch targets ≥40px; composer textarea bumped to 16px so iOS Safari doesn't
  auto-zoom on focus (which itself shoved the composer out of view).

Verified at 390×844 (iPhone) and 1280×800 via Playwright: input box renders at
y=723–821 (inside the 844 viewport), sheet slides in on tap, desktop keeps the
2-column side panel.

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

frontend/src/Chat.svelte

@@ -443,14 +443,16 @@
     flex: 1;
     resize: none;
     max-height: 168px;
-    min-height: 44px;
+    min-height: 48px;
     background: var(--bg-2);
     color: var(--ink);
     border: 1px solid var(--line-strong);
     border-radius: var(--radius-sm);
-    padding: 11px 13px;
+    padding: 12px 13px;
     font-family: var(--sans);
-    font-size: 14px;
+    /* 16px: anything smaller makes iOS Safari auto-zoom on focus (mobile is the
+       primary client) — the zoom then shifts the composer out of view. */
+    font-size: 16px;
     line-height: 1.5;
     outline: none;
     transition: border-color 0.15s, box-shadow 0.15s;