fix: Fix review showing empty files + propagate diff errors to UI (#200)

* 🐛 fix: force standard a/b diff prefix to handle mnemonic prefix config

When diff.mnemonicPrefix is enabled in git config, `git diff` uses
context-dependent prefixes (c/ for commit, w/ for worktree, i/ for
index) instead of the standard a/b. Both the internal parseDiffToFiles
and @pierre/diffs library expect a/b prefixes, causing silent parse
failures that result in an empty file list.

Add --src-prefix=a/ --dst-prefix=b/ to all git diff invocations to
ensure consistent output regardless of user git configuration.

* ✨ feat: propagate git diff errors to review UI

Previously, git diff errors were silently caught and returned as empty
patches. The UI showed "No changes" with no indication of failure.

- Add error field to DiffResult and propagate through ReviewServerOptions
- Include error in /api/diff and /api/diff/switch responses
- Show distinct error state in review UI (red icon + error message)
- Clear/set error state on diff type switch

* Fix mnemonic prefix handling in pi-extension runGitDiff

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Michael Ramos <mdramos8@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/server/review.ts

@@ -28,6 +28,8 @@ export interface ReviewServerOptions {
   rawPatch: string;
   /** Git ref used for the diff (e.g., "HEAD", "main..HEAD", "--staged") */
   gitRef: string;
+  /** Error message if git diff failed */
+  error?: string;
   /** HTML content to serve for the UI */
   htmlContent: string;
   /** Origin identifier for UI customization */
@@ -89,6 +91,7 @@ export async function startReviewServer(
   let currentPatch = options.rawPatch;
   let currentGitRef = options.gitRef;
   let currentDiffType: DiffType = options.diffType || "uncommitted";
+  let currentError = options.error;
 
   const isRemote = isRemoteSession();
   const configuredPort = getServerPort();
@@ -132,6 +135,7 @@ export async function startReviewServer(
               sharingEnabled,
               shareBaseUrl,
               repoInfo,
+              ...(currentError && { error: currentError }),
             });
           }
 
@@ -156,11 +160,13 @@ export async function startReviewServer(
               currentPatch = result.patch;
               currentGitRef = result.label;
               currentDiffType = newDiffType;
+              currentError = result.error;
 
               return Response.json({
                 rawPatch: currentPatch,
                 gitRef: currentGitRef,
                 diffType: currentDiffType,
+                ...(currentError && { error: currentError }),
               });
             } catch (err) {
               const message =