feat(ai): preview button strings and CSS for edit summary

Add AI_CHAT_PREVIEW_OPEN and AI_CHAT_PREVIEW_VIEWING i18n strings.
Add .ai-edit-preview-btn styles with active/hover states for the
live preview toggle button in edit summaries.

Author: abose

src/nls/root/strings.js

@@ -1931,6 +1931,8 @@ define({
     "AI_CHAT_CODE_EXPAND": "Expand",
     "AI_CHAT_CODE_EXPAND_LINES": "Click to expand - {0} lines",
     "AI_CHAT_CODE_COLLAPSE_FOOTER": "Click to collapse",
+    "AI_CHAT_PREVIEW_OPEN": "Preview",
+    "AI_CHAT_PREVIEW_VIEWING": "Previewing",
     "AI_CHAT_QUESTION_OTHER": "Type a custom answer\u2026",
     "AI_CHAT_IMAGE_LIMIT": "Maximum {0} images allowed",
     "AI_CHAT_IMAGE_REMOVE": "Remove image",

src/styles/Extn-AIChatPanel.less

@@ -847,6 +847,42 @@
     }
 }
 
+.ai-edit-preview-btn {
+    display: flex;
+    align-items: center;
+    gap: 4px;
+    margin-top: 6px;
+    background: rgba(102, 187, 106, 0.1);
+    border: 1px solid rgba(102, 187, 106, 0.25);
+    color: #66bb6a;
+    font-size: @sidebar-small-font-size;
+    padding: 4px 12px;
+    border-radius: 4px;
+    cursor: pointer;
+    transition: background-color 0.15s ease, border-color 0.15s ease;
+
+    &:hover:not(.active) {
+        background: rgba(102, 187, 106, 0.18);
+        border-color: rgba(102, 187, 106, 0.4);
+    }
+
+    &.active {
+        background: rgba(102, 187, 106, 0.15);
+        border-color: rgba(102, 187, 106, 0.3);
+        color: #66bb6a;
+        cursor: pointer;
+
+        &:hover {
+            background: rgba(102, 187, 106, 0.1);
+            border-color: rgba(102, 187, 106, 0.2);
+        }
+    }
+
+    i {
+        font-size: 11px;
+    }
+}
+
 /* ── Initial restore point (PUC) ────────────────────────────────────── */
 .ai-msg-restore-point {
     display: flex;