feat(frontend): rebrand Sub2API to FireflyAPI

- Replace all user-visible Sub2API brand names with FireflyAPI
- Remove GitHub navigation link from AppHeader dropdown
- Remove GitHub link from HomeView footer
- Simplify VersionBadge to static version display (remove update check)
- Update i18n zh/en locale strings
- Update default site name fallbacks across all views

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

frontend/src/views/setup/SetupWizardView.vue

@@ -131,7 +131,7 @@
                 v-model="formData.database.dbname"
                 type="text"
                 class="input"
-                placeholder="sub2api"
+                placeholder="fireflyapi"
               />
             </div>
             <div>
@@ -535,7 +535,7 @@ const formData = reactive<InstallRequest>({
     port: 5432,
     user: 'postgres',
     password: '',
-    dbname: 'sub2api',
+    dbname: 'fireflyapi',
     sslmode: 'disable'
   },
   redis: {