feat: phone setup QR code in dashboard header

- Add /qr endpoint serving SVG (uses existing qrcode crate, svg feature)
- Header popover: QR on desktop, direct download link on mobile viewports
- Only visible when [mobile] enabled = true in config
- Expose mobile.enabled and mobile.port in /stats response
- Lazy-load QR on first click, dismiss on outside click

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -30,7 +30,7 @@ tokio-rustls = "0.26"
 arc-swap = "1"
 ring = "0.17"
 rustls-pemfile = "2.2.0"
-qrcode = { version = "0.14", default-features = false }
+qrcode = { version = "0.14", default-features = false, features = ["svg"] }
 
 [dev-dependencies]
 criterion = { version = "0.8", features = ["html_reports"] }