feat(rules): the edit history is visible in the slide-over (#3243, milestone 323 step 4)

Rule 27: a history nobody can read is not shipped. `RuleHistoryPanel.vue`
sits below the fields in `RuleEditorSlideOver`, where a rule is read in full
— not on the list row, where a history entry point would compete with the
row's job.

REUSE, DECIDED FIELD BY FIELD RATHER THAN ALL AT ONCE.

DiffView.vue is reused unchanged: it takes DiffLine[] and nothing
note-shaped.

HistoryPanel.vue is NOT, and its props are the reason — noteId +
currentBody, a NoteVersion carrying tags and pin columns, a fetch of
/api/notes/…, a restore emit, pin/unpin buttons. Rules have no tags, no
pins, and deliberately no restore, and a rule's text is EIGHT fields rather
than one body, which changes the reader's question from "what changed" to
"which fields moved". Recorded here rather than forked silently, per #3207.

THE FORK THAT WAS ALREADY THERE. The LCS walk existed three times —
privately in useAssist.ts, and again inside HistoryPanel.vue and
VersionHistorySection.vue — character-identical apart from quote style,
because computeDiff was never exported. Rather than add a fourth copy, it
moves to utils/diff.ts and the three become imports; the extraction was
verified equivalent to all three before anything was deleted. DiffLine is
re-exported from useAssist so its existing importers are untouched.

WHAT A ROW SHOWS: when, and which fields moved. A version holds the text the
edit REPLACED, so the edit is the step from a row to the next NEWER state —
the row above it, or, for the newest row, the rule as it stands now.
Comparing against the row below would attribute every change to the wrong
edit. A field nobody has fetched yet reads as neither changed nor unchanged.

An edit that touched verify_with is badged "check reset", because that edit
silently cleared verified_at (milestone 312) and put the rule back at the
top of the staleness sweep — a moment visible nowhere else.

The badge is a 12% color-mix TINT, not solid `--fs-warning`.
`--fs-warning-fg` is defined in theme.css as "warning TEXT on a warning
tint", so painting it over the solid token is exactly the same-hue contrast
failure #3141 records. Every var() the component references resolves against
theme.css, checked before pushing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/components/HistoryPanel.vue

@@ -2,7 +2,7 @@
 import { ref, computed, onMounted } from "vue";
 import { apiGet, pinNoteVersion, unpinNoteVersion } from "@/api/client";
 import DiffView from "@/components/DiffView.vue";
-import type { DiffLine } from "@/composables/useAssist";
+import { computeDiff, type DiffLine } from "@/utils/diff";
 import { fmtStamp } from "@/utils/dateFormat";
 
 interface NoteVersion {
@@ -33,28 +33,8 @@ const loadingDetail = ref(false);
 
 const diff = computed<DiffLine[]>(() => {
   if (!selectedVersion.value?.body) return [];
-  const a = props.currentBody;
-  const b = selectedVersion.value.body;
 
-  const aLines = a.split('\n');
-  const bLines = b.split('\n');
-  const m = aLines.length, n = bLines.length;
-  const dp: number[][] = Array.from({ length: m + 1 }, () => new Array(n + 1).fill(0));
-  for (let i = m - 1; i >= 0; i--)
-    for (let j = n - 1; j >= 0; j--)
-      dp[i][j] = aLines[i] === bLines[j]
-        ? dp[i+1][j+1] + 1
-        : Math.max(dp[i+1][j], dp[i][j+1]);
-  const result: DiffLine[] = [];
-  let i = 0, j = 0;
-  while (i < m && j < n) {
-    if (aLines[i] === bLines[j]) { result.push({ type: 'equal', text: aLines[i++] }); j++; }
-    else if (dp[i+1][j] >= dp[i][j+1]) result.push({ type: 'delete', text: aLines[i++] });
-    else result.push({ type: 'insert', text: bLines[j++] });
-  }
-  while (i < m) result.push({ type: 'delete', text: aLines[i++] });
-  while (j < n) result.push({ type: 'insert', text: bLines[j++] });
-  return result;
+  return computeDiff(props.currentBody, selectedVersion.value.body);
 });
 
 async function loadVersions() {