feat(ai): disable local provider and improve commit file display

Local AI is now treated as unavailable in the settings UI, with a
migration to ensure any previously saved "local" selection switches
back to OpenAI. The history panel also improves how commit file
names are presented, including clearer old->new path formatting.

- Migrate stored AI provider away from local to prevent dead state
- Disable local provider option with an "in development" badge
- Refine history panel filename rendering and tooltip context

src/lib/components/HistoryPanel.svelte

@@ -113,6 +113,16 @@
     return file.old_path ? `${file.old_path} -> ${file.path}` : file.path;
   }
 
+  function baseName(path: string): string {
+    return path.split(/[\\/]/).filter(Boolean).pop() ?? path;
+  }
+
+  function commitFileName(file: GitCommitFile): string {
+    return file.old_path
+      ? `${baseName(file.old_path)} -> ${baseName(file.path)}`
+      : baseName(file.path);
+  }
+
   function formatCommitDate(value: string): string {
     const date = new Date(value);
     if (Number.isNaN(date.getTime())) return value;
@@ -208,10 +218,10 @@
                         type="button"
                         onclick={() => onPreviewCommitFile(item, file)}
                         disabled={isBusy}
-                        title="Show differences before restoring"
+                        title={`Show differences before restoring - ${displayCommitFile(file)}`}
                       >
                         <span class={`status-badge ${file.status}`}>{statusLabel(file.status)}</span>
-                        <strong>{displayCommitFile(file)}</strong>
+                        <strong>{commitFileName(file)}</strong>
                       </button>
                     {/each}
                   </div>