feat(desktop): PR-style file diffs in chat

Render write_file/edit_file/patch as a reviewable diff instead of raw
result JSON, closer to a Cursor/T3 per-edit review.

- Unified diff via FileDiffPanel: strip git file-header + @@ hunk noise,
  drop the +/- gutter, color by line with a 2px gutter accent, full-bleed
  to the card, transparent context lines, compact scroll height.
- Header shows filename + language icon + +N/-N stats; full path moves to
  a hover tooltip (no Edited verb, no ms).
- Treat the three file-edit tools uniformly (isFileEditTool); read diff
  from inline_diff or patch's diff field; suppress raw-arg detail.
- Reusable FileTypeIcon primitive sharing the code-block icon mapping
  (codiconForFilename), codicon fallback.
- Per-row scaffolding fade (not the group wrapper, which trapped child
  opacity); expanded edits stay full, collapsed fade; keyboard-only focus
  lift. Hide diff-less rehydrated creates that read as dupes.

apps/desktop/src/components/chat/diff-lines.tsx

@@ -15,11 +15,17 @@ interface DiffLineKind {
 
 const DIFF_LINE_KINDS: DiffLineKind[] = [
   {
-    className: 'text-emerald-700 dark:text-emerald-300',
+    className: 'border-emerald-500 bg-emerald-500/12 text-emerald-800 dark:text-emerald-200',
     match: line => line.startsWith('+') && !line.startsWith('+++')
   },
-  { className: 'text-rose-700 dark:text-rose-300', match: line => line.startsWith('-') && !line.startsWith('---') },
-  { className: 'text-sky-700 dark:text-sky-300', match: line => line.startsWith('@@') },
+  {
+    className: 'border-rose-500 bg-rose-500/12 text-rose-800 dark:text-rose-200',
+    match: line => line.startsWith('-') && !line.startsWith('---')
+  },
+  {
+    className: 'text-sky-700 dark:text-sky-300',
+    match: line => line.startsWith('@@')
+  },
   {
     className: 'text-muted-foreground/70',
     match: line => line.startsWith('---') || line.startsWith('+++') || / → /.test(line.slice(0, 60))
@@ -30,25 +36,127 @@ function classifyLine(line: string): string | undefined {
   return DIFF_LINE_KINDS.find(kind => kind.match(line))?.className
 }
 
+// Drop the leading +/-/space gutter character so changes read by color alone
+// (like Cursor), keeping the rest of the indentation intact. Hunk headers
+// (`@@`) and any stray file headers are left untouched.
+function stripDiffMarker(line: string): string {
+  if (line.startsWith('@@')) {
+    return line
+  }
+
+  if ((line.startsWith('+') && !line.startsWith('+++')) || (line.startsWith('-') && !line.startsWith('---'))) {
+    return line.slice(1)
+  }
+
+  if (line.startsWith(' ')) {
+    return line.slice(1)
+  }
+
+  return line
+}
+
+interface DisplayLine {
+  className?: string
+  text: string
+}
+
+// Build the rendered line list: drop `@@ … @@` hunk headers (git noise in a
+// GUI) and the +/- gutter, but keep a blank separator between hunks so
+// multi-hunk diffs don't visually merge.
+function toDisplayLines(text: string): DisplayLine[] {
+  const out: DisplayLine[] = []
+  let emitted = false
+
+  for (const line of text.split('\n')) {
+    if (line.startsWith('@@')) {
+      if (emitted) {
+        out.push({ text: '' })
+      }
+
+      continue
+    }
+
+    out.push({ className: classifyLine(line), text: stripDiffMarker(line) })
+    emitted = true
+  }
+
+  return out
+}
+
 interface DiffLinesProps extends Omit<React.ComponentProps<'pre'>, 'children'> {
   text: string
 }
 
 export function DiffLines({ className, text, ...props }: DiffLinesProps) {
+  const lines = React.useMemo(() => toDisplayLines(text), [text])
+
   return (
     <pre
       className={cn(
-        'mt-1 mb-1.5 max-h-96 max-w-full min-w-0 overflow-auto rounded-md border border-border/60 bg-muted/35 px-2.5 py-1.5 font-mono text-[0.7rem] leading-relaxed text-muted-foreground',
+        'max-h-[12rem] max-w-full min-w-0 overflow-auto overscroll-contain px-0 py-1 font-mono text-[0.7rem] leading-relaxed text-(--ui-text-secondary)',
         className
       )}
       data-slot="diff-lines"
       {...props}
     >
-      {text.split('\n').map((line, index) => (
-        <span className={cn('block min-w-max whitespace-pre', classifyLine(line))} key={`${index}-${line}`}>
-          {line || ' '}
+      {lines.map((line, index) => (
+        <span
+          className={cn('block min-w-max border-l-2 border-transparent whitespace-pre px-2.5 py-px', line.className)}
+          key={`${index}-${line.text}`}
+        >
+          {line.text || ' '}
         </span>
       ))}
     </pre>
   )
 }
+
+// Git-style unified diffs arrive with a file-header preamble — `diff --git`,
+// `index …`, `--- a/path`, `+++ b/path`, and Hermes' own `a/path → b/path`
+// arrow line. That preamble just repeats the path (which the tool row already
+// shows) and reads especially badly for absolute paths (`a//Users/…`). Strip
+// the leading header zone up to the first hunk so the panel shows only hunks +
+// changes, the way Cursor does.
+const DIFF_HEADER_PREFIXES = ['diff --git', 'index ', '--- ', '+++ ', 'similarity ', 'rename ', 'new file', 'deleted file']
+
+function isArrowHeaderLine(line: string): boolean {
+  const trimmed = line.trim()
+
+  return trimmed.includes('→') && /^\S.*→\s*\S+$/.test(trimmed) && !/^[+\-@]/.test(trimmed)
+}
+
+/** Exported for tests. */
+export function stripDiffFileHeaders(diff: string): string {
+  const lines = diff.split('\n')
+  let start = 0
+
+  for (; start < lines.length; start += 1) {
+    const line = lines[start]
+
+    if (line.startsWith('@@')) {
+      break
+    }
+
+    if (line.trim() === '' || isArrowHeaderLine(line) || DIFF_HEADER_PREFIXES.some(prefix => line.startsWith(prefix))) {
+      continue
+    }
+
+    break
+  }
+
+  return lines.slice(start).join('\n')
+}
+
+interface FileDiffPanelProps {
+  diff: string
+}
+
+export function FileDiffPanel({ diff }: FileDiffPanelProps) {
+  const display = React.useMemo(() => stripDiffFileHeaders(diff), [diff])
+
+  // Bleed out of the tool-card body's `p-1.5` so changed-line tints/borders run
+  // flush to the card edges (rounded corners clip via the card's overflow).
+  // `max-w-none` lifts the base `max-w-full` cap that would otherwise stop the
+  // negative margins from widening the block.
+  return <DiffLines className="-mx-1.5 -mb-1.5 max-w-none" data-slot="file-diff-panel" text={display} />
+}