fix(diff): correctly classify file type for addition-only modifications

The diff viewer was not showing green background highlighting for added
lines in modified files that only had additions (no deletions), like
`src/constants.js` in svelte 5.16.2→5.50.0.

Root cause:
1. `parseUnifiedDiff` used a heuristic that reclassified files based on
   diff content: if a file had only insertions, it was marked as 'add'
   (new file). This was wrong for modifications with only additions.
2. `computeDiff` in ViewerPanel used `parsed.type` ('add') instead of
   the correctly computed `type` ('modify') based on file existence.
3. DiffLine intentionally skips green background when fileStatus='add'
   (all lines in a new file are additions, so highlighting is redundant).
   This caused addition-only modifications to lose visual highlighting.

Fix:
- `parseUnifiedDiff`: Replace the content-based heuristic with proper
  /dev/null path detection from --- / +++ headers, which is the standard
  way unified diffs indicate new/deleted files.
- `ViewerPanel.vue`: Use the file-existence-based type instead of the
  parsed diff type.

Author: taskylizard

app/components/diff/ViewerPanel.vue

@@ -119,7 +119,11 @@ function computeDiff() {
     from: props.fromVersion,
     to: props.toVersion,
     path: props.file.path,
-    type: parsed.type,
+    // Use the type computed from file existence (modify/add/delete) rather than
+    // parsed.type which incorrectly classifies addition-only modifications as 'add'.
+    // This matters because DiffLine skips green background highlighting when
+    // fileStatus is 'add' (since for truly new files all lines are additions).
+    type,
     hunks: hunksWithSkips,
     stats,
     meta: {},

shared/utils/diff.ts

@@ -183,19 +183,24 @@ export function parseUnifiedDiff(
   let oldLine = 0
   let newLine = 0
 
+  // Track old path from --- line to detect /dev/null (new/deleted files)
+  let lastOldPath = ''
+
   for (const line of lines) {
     if (line.startsWith('---')) {
       if (currentHunk && currentFile) {
         const hunk = processHunk(currentHunk, opts)
         currentFile.hunks.push(hunk)
       }
       currentHunk = null
+      const oldMatch = line.match(/^--- (?:a\/)?(.*)/)
+      lastOldPath = oldMatch?.[1]?.trimEnd() ?? ''
       continue
     }
 
     if (line.startsWith('+++')) {
       const match = line.match(/^\+\+\+ (?:b\/)?(.*)/)
-      const path = match?.[1] ?? ''
+      const path = match?.[1]?.trimEnd() ?? ''
 
       if (currentFile && currentHunk) {
         const hunk = processHunk(currentHunk, opts)
@@ -205,13 +210,20 @@ export function parseUnifiedDiff(
         files.push(currentFile)
       }
 
+      // Determine file type from --- / +++ paths:
+      // /dev/null in the old path means a new file; in the new path means deleted
+      let fileType: FileDiff['type'] = 'modify'
+      if (lastOldPath === '/dev/null') fileType = 'add'
+      else if (path === '/dev/null') fileType = 'delete'
+
       currentFile = {
-        oldPath: path,
+        oldPath: lastOldPath === '/dev/null' ? path : lastOldPath || path,
         newPath: path,
-        type: 'modify',
+        type: fileType,
         hunks: [],
       }
       currentHunk = null
+      lastOldPath = ''
       continue
     }
 
@@ -279,19 +291,6 @@ export function parseUnifiedDiff(
     files.push(currentFile)
   }
 
-  for (const file of files) {
-    const hasAdds = file.hunks.some(
-      h => h.type === 'hunk' && h.lines.some(l => l.type === 'insert'),
-    )
-    const hasDels = file.hunks.some(
-      h => h.type === 'hunk' && h.lines.some(l => l.type === 'delete'),
-    )
-
-    if (hasAdds && !hasDels) file.type = 'add'
-    else if (hasDels && !hasAdds) file.type = 'delete'
-    else file.type = 'modify'
-  }
-
   return files
 }