fix: add back-to-editor navigation from commit diff view

When viewing a commit diff from git history, the GitBranch toggle in the
breadcrumb bar was disabled if the file had no unstaged changes, leaving
no way to return to the editor. Now the toggle stays active whenever
diffMode is true, and a visible "Back to editor" button appears at the
top of the diff view for better discoverability.

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

src/components/Editor.tsx

@@ -356,7 +356,7 @@ export const Editor = memo(function Editor({
 
   const activeTab = tabs.find((t) => t.entry.path === activeTabPath) ?? null
   const isLoadingNewTab = activeTabPath !== null && !activeTab
-  const showDiffToggle = activeTab && isModified?.(activeTab.entry.path)
+  const showDiffToggle = activeTab && (diffMode || isModified?.(activeTab.entry.path))
 
   useEffect(() => {
     setDiffMode(false)
@@ -486,6 +486,14 @@ export const Editor = memo(function Editor({
           {breadcrumbBar}
           {diffMode && (
             <div className="flex-1 overflow-auto">
+              <button
+                className="flex items-center gap-1.5 px-4 py-2 text-xs text-primary bg-muted border-b border-border cursor-pointer hover:bg-accent transition-colors w-full border-t-0 border-l-0 border-r-0"
+                onClick={handleToggleDiff}
+                title="Back to editor"
+              >
+                <span style={{ fontSize: 14, lineHeight: 1 }}>&larr;</span>
+                Back to editor
+              </button>
               <DiffView diff={diffContent ?? ''} />
             </div>
           )}