feat(FN-1493): add directory browsing UX to plugin install flow

- Add DirectoryPicker component for browsing filesystem to select plugin source
- Add dist-folder and package-root install-source resolution in backend
- Wire frontend installPlugin to mode-discriminated endpoint (/plugins/install-local)
- Add comprehensive regression tests for browse-driven plugin install workflow
- Update PluginManager UI with directory picker integration and CSS styles

packages/dashboard/app/api.ts

@@ -4627,9 +4627,9 @@ export async function installPlugin(
   source: { path: string } | { package: string },
   projectId?: string,
 ): Promise<PluginInstallation> {
-  return api<PluginInstallation>(withProjectId("/plugins/install", projectId), {
+  return api<PluginInstallation>(withProjectId("/plugins", projectId), {
     method: "POST",
-    body: JSON.stringify(source),
+    body: JSON.stringify({ mode: "install", ...source }),
   });
 }