replace client-side tab toggling with server-side tab routing

Each settings tab is now a separate page load via ?tab= query param.
Flask renders only the active panel - no CSS or JS visibility logic needed.
Tab bar uses <a> links instead of buttons. Eliminates the display:none
cascade issue that was preventing Import and Maintenance tabs from showing.

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

app/main.py

@@ -594,7 +594,11 @@ def artist_gallery(artist_name):
 
 @main.route('/settings')
 def settings():
-    return render_template('settings.html')
+    valid_tabs = {'overview', 'import', 'maintenance'}
+    tab = request.args.get('tab', 'overview')
+    if tab not in valid_tabs:
+        tab = 'overview'
+    return render_template('settings.html', active_tab=tab)
 
 
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