feat(ux): translate becomes a primary FAB stacked under camera

Promotes Translate from an inline button under the input pane to
the primary 56x56 FAB at the bottom-right corner. Camera FAB
becomes the secondary in the stack, just above. Both share a
single .fab base class; positioning via .fab-primary /
.fab-secondary.

- Translate icon: Material Symbols `translate` (28 px).
- Pulsing-ring `busy` animation while a translation runs, since
  the icon-only button can't show "Translating…" text anymore.
- body padding-bottom bumped to ~9 rem so the output pane
  scrolls clear of the stacked FABs.
- ⌘/Ctrl-Enter still triggers translate via the existing
  CodeMirror keymap; tooltip text updated.

static/index.html

@@ -34,7 +34,8 @@
       background: var(--bg);
       color: var(--fg);
       margin: 0;
-      padding: 2rem 1rem;
+      /* extra bottom padding so content scrolls clear of the stacked FABs */
+      padding: 2rem 1rem calc(9rem + env(safe-area-inset-bottom, 0px)) 1rem;
     }
     .wrap { max-width: 960px; margin: 0 auto; }
     h1 {
@@ -165,10 +166,10 @@
       user-select: none;
     }
 
-    /* Floating action button for OCR scan — bottom-right, mobile-thumb friendly */
-    .scan-fab {
+    /* Floating action buttons — stacked at bottom-right.
+       Translate is the primary action (closest to the thumb), Scan sits above. */
+    .fab {
       position: fixed;
-      bottom: calc(1.5rem + env(safe-area-inset-bottom, 0px));
       right: calc(1.5rem + env(safe-area-inset-right, 0px));
       width: 56px;
       height: 56px;
@@ -184,11 +185,25 @@
       display: flex;
       align-items: center;
       justify-content: center;
-      transition: transform 0.15s;
+      transition: transform 0.15s, box-shadow 0.15s;
+    }
+    .fab .material-symbols-outlined { font-size: 28px; }
+    .fab:hover:not(:disabled) { transform: scale(1.06); }
+    .fab:disabled { opacity: 0.55; cursor: progress; }
+    .fab-primary {
+      bottom: calc(1.5rem + env(safe-area-inset-bottom, 0px));
+    }
+    .fab-secondary {
+      /* sits above the primary: 56 (primary height) + 0.75rem gap + base offset */
+      bottom: calc(1.5rem + 56px + 0.75rem + env(safe-area-inset-bottom, 0px));
+    }
+    .fab.busy {
+      animation: fab-pulse 1.4s ease-in-out infinite;
+    }
+    @keyframes fab-pulse {
+      0%, 100% { box-shadow: 0 4px 14px rgba(0, 0, 0, 0.35); }
+      50%      { box-shadow: 0 4px 14px rgba(0, 0, 0, 0.35), 0 0 0 8px rgba(224, 122, 62, 0.35); }
     }
-    .scan-fab .material-symbols-outlined { font-size: 28px; }
-    .scan-fab:hover:not(:disabled) { transform: scale(1.06); }
-    .scan-fab:disabled { opacity: 0.6; cursor: progress; }
     .scan-status {
       font-size: 0.75rem;
       color: var(--muted);
@@ -408,13 +423,15 @@
       </div>
     </div>
 
-    <button id="go">Translate</button>
     <div id="meta" class="meta"></div>
   </div>
 
-  <button type="button" id="scan-btn" class="scan-fab" aria-label="Scan image (OCR)" title="Scan image (OCR)">
+  <button type="button" id="scan-btn" class="fab fab-secondary" aria-label="Scan image (OCR)" title="Scan image (OCR)">
     <span class="material-symbols-outlined">photo_camera</span>
   </button>
+  <button type="button" id="go" class="fab fab-primary" aria-label="Translate" title="Translate (⌘/Ctrl + Enter)">
+    <span class="material-symbols-outlined">translate</span>
+  </button>
 
   <script type="module">
     import { EditorView, keymap, placeholder } from "https://esm.sh/@codemirror/view@6";
@@ -935,7 +952,7 @@
       const text = inputText().trim();
       if (!text) return;
       goBtn.disabled = true;
-      goBtn.textContent = "Translating…";
+      goBtn.classList.add("busy");
       output.textContent = "";
       output.classList.add("streaming");
 
@@ -1007,7 +1024,7 @@
       } finally {
         output.classList.remove("streaming");
         goBtn.disabled = false;
-        goBtn.textContent = "Translate";
+        goBtn.classList.remove("busy");
       }
     }