feat: add copy-to-clipboard button on code blocks in viewer + popout

- CSS: .code-block-wrapper with relative positioning
- CSS: .copy-code-btn with hover opacity transition (hidden until hover)
- CSS: .copied state (green accent) after successful copy
- viewer.js: wraps each <pre> in .code-block-wrapper + appends copy button
- popout.html: same logic with vanilla JS (ES5 compatible)
- Button: 'Copier' → '✓ Copié' on click, reverts after 2s
- Toast: showToast('Code copié dans le presse-papiers') via navigator.clipboard

frontend/style.css

@@ -8528,3 +8528,35 @@ body.popup-mode .content-area {
 .pane-resize-handle:hover {
   background: var(--accent);
 }
+.code-block-wrapper:hover .copy-code-btn {
+  opacity: 1;
+}
+.copy-code-btn {
+  position: absolute;
+  top: 8px;
+  right: 8px;
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  padding: 4px 10px;
+  background: var(--bg-secondary);
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  color: var(--text-secondary);
+  font-size: 11px;
+  font-family: var(--mono);
+  cursor: pointer;
+  opacity: 0;
+  transition: opacity 150ms ease, background 150ms ease, color 150ms ease;
+  z-index: 5;
+}
+.copy-code-btn:hover {
+  background: var(--accent);
+  color: #fff;
+  border-color: var(--accent);
+}
+.copy-code-btn.copied {
+  background: var(--accent-green);
+  color: #fff;
+  border-color: var(--accent-green);
+}