feat: add auto-update checker

Implements issue #17 - the app now checks for updates on startup and shows
a notification when a newer version is available. Users can disable this
in settings. Uses Gitea releases API with semver comparison.

Closes #17

src/routes/+page.svelte

@@ -18,8 +18,10 @@
   import ConfigSidebar from "$lib/components/ConfigSidebar.svelte";
   import AchievementNotification from "$lib/components/AchievementNotification.svelte";
   import AchievementsPanel from "$lib/components/AchievementsPanel.svelte";
+  import UpdateNotification from "$lib/components/UpdateNotification.svelte";
 
   let initialized = false;
+  let updateNotification: UpdateNotification;
   let achievementPanelOpen = $state(false);
 
   // Global keyboard shortcuts
@@ -108,6 +110,11 @@
 
       // Add global keyboard shortcut listener
       window.addEventListener("keydown", handleGlobalKeydown);
+
+      // Check for updates on startup
+      if (config.update_checks_enabled) {
+        updateNotification?.checkForUpdates();
+      }
     }
   });
 
@@ -147,6 +154,7 @@
     bind:isOpen={achievementPanelOpen}
     onClose={() => (achievementPanelOpen = false)}
   />
+  <UpdateNotification bind:this={updateNotification} />
 </div>
 
 <style>