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.

CHANGELOG.md

@@ -16,6 +16,7 @@ Versioning: [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Changed
 - "Scan image" button replaced with a floating action button at the bottom-right of the viewport, using a Material Symbols Outlined camera glyph (`photo_camera`) for consistent rendering across platforms (was a small text button in the input pane header).
+- "Translate" inline button replaced with a primary FAB using the `translate` Material Symbols icon, stacked directly under the camera FAB at the bottom-right corner (closest to the thumb on phones). The camera FAB is now the *secondary* in the stack, just above. Both 56 × 56 px, same accent fill. The translate FAB gains a pulsing-ring `busy` animation during a running translation; `goBtn.textContent` swap is gone (was meaningless for an icon-only button). `body` got `~9 rem` of bottom padding so content scrolls clear of the stack.
 - Crop modal:
   - No default crop box. User drags on the image to draw the region; falls back to full image if they confirm without drawing.
   - Buttons are now Material Symbols icons: `rotate_left`, `rotate_right`, `invert_colors` (new — see below), `check`. Cancel kept as text. The `check` (Use this) button is larger and more prominent. The Reset button was removed (rarely useful with no default box).