Make onboarding tour install-wide instead of per-browser

Persist onboardingCompleted flag server-side via /api/v1/config so the
tour only auto-starts once per DashCaddy installation, not on every
new browser that connects.

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

status/js/tour-manager.js

@@ -70,7 +70,8 @@
      * Check if tour should auto-start
      */
     shouldAutoStart() {
-      return !this.progressTracker.isTourCompleted() && 
+      return !this.progressTracker.isInstallOnboardingCompleted() &&
+             !this.progressTracker.isTourCompleted() && 
              this.progressTracker.getCurrentStep() === 0;
     }