feat: add file edit diff progress view

Capture file edit snapshots through runner tool lifecycle hooks and render unified diffs in the WebUI with folding and truncation controls.

webui/src/i18n/locales/en/common.json

@@ -141,6 +141,7 @@
       "presetModel": "Preset model",
       "density": "Density",
       "activityMode": "Activity detail",
+      "fileEditDisplay": "File edit display",
       "codeWrap": "Code wrapping",
       "brandLogos": "Brand logos",
       "maxResults": "Max results",
@@ -187,6 +188,7 @@
       "presetModel": "Switch to Default to edit model and provider from the WebUI.",
       "density": "Stored only in this browser.",
       "activityMode": "Choose how much agent activity chrome to show by default.",
+      "fileEditDisplay": "Choose whether file edit activity opens as line counts or a diff.",
       "codeWrap": "Keep long code lines readable on smaller screens.",
       "brandLogos": "Show third-party provider and CLI logos in Settings.",
       "maxResults": "Results returned by each web_search call.",
@@ -329,6 +331,9 @@
       "compact": "Compact",
       "auto": "Auto",
       "expanded": "Expanded",
+      "summary": "Summary",
+      "diff": "Diff",
+      "collapsedDiff": "Collapsed diff",
       "on": "On",
       "off": "Off",
       "defaultPermission": "Default Permission",
@@ -1038,7 +1043,15 @@
     "forkFromHere": "Fork",
     "copyReply": "Copy",
     "copiedReply": "Copied",
-    "turnLatencyTitle": "Response time (end-to-end)"
+    "turnLatencyTitle": "Response time (end-to-end)",
+    "fileEditViewDiff": "View diff",
+    "fileEditViewLargeDiff": "View large diff",
+    "fileEditDiffLineCount": "{{count}} lines",
+    "fileEditUnchangedLinesHidden": "{{count}} unchanged lines hidden",
+    "fileEditShowMoreLines": "Show {{count}} more lines",
+    "fileEditShowFewerLines": "Show fewer lines",
+    "fileEditOpenFile": "Open file",
+    "fileEditDiffTruncated": "Diff truncated. Open the file for the full change."
   },
   "lightbox": {
     "title": "Image preview",