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/static/style.css

@@ -1233,6 +1233,8 @@ header {
 }
 
 .settings-tab-btn {
+  display: inline-block;
+  text-decoration: none;
   padding: 0.4rem 1.1rem;
   border-radius: 999px;
   border: 1px solid var(--filter-border);
@@ -1253,12 +1255,8 @@ header {
   color: var(--filter-active-fg);
 }
 
-/* Settings tab panels */
+/* Settings tab panels — always visible, server renders only the active one */
 .settings-tab-panel {
-  display: none;
-}
-
-.settings-tab-panel.is-active {
   display: block;
 }