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-tauri/src/lib.rs

@@ -120,6 +120,8 @@ pub fn run() {
             commands::update_commands::get_app_version,
             commands::update_commands::check_for_updates,
             commands::update_commands::check_image_update,
+            // Help
+            commands::help_commands::get_help_content,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");