Fetch help content from repo, add TOC and marketplace troubleshooting

Help dialog now fetches HOW-TO-USE.md live from the gitea repo on open,
falling back to the compile-time embedded copy when offline. Content is
cached for the session. Removes the ~600-line hardcoded markdown constant
from HelpDialog.tsx in favor of a single source of truth.

Adds a Table of Contents with anchor links for quick navigation and a new
troubleshooting entry for the "Failed to install Anthropic marketplace"
error with the jq fix. Markdown renderer updated to support anchor links
and header id attributes.

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

app/src/lib/tauri-commands.ts

@@ -85,3 +85,6 @@ export const checkForUpdates = () =>
   invoke<UpdateInfo | null>("check_for_updates");
 export const checkImageUpdate = () =>
   invoke<ImageUpdateInfo | null>("check_image_update");
+
+// Help
+export const getHelpContent = () => invoke<string>("get_help_content");