Add OIDC desktop callback, app logo, rename to Oikos

- Server: /oidc-callback HTML page exchanges Authentik code for token,
  displays it for user to copy into the desktop app's Token tab
- oidc.ts: desktop mode uses apiUrl+/oidc-callback as redirect URI,
  encodes PKCE verifier in state parameter
- Config.svelte: add Server URL field to OIDC tab for desktop UX
- Caddy: add /oidc-callback to enroll bypass (no Authentik gate)
- App: favicon.png as system tray icon, window title 'Oikos'
- web/index.html: title 'Oikos'

compose/caddy/Caddyfile.oikos

@@ -21,7 +21,7 @@ oikos.hubris.network {
     tls {
         dns ionos {env.IONOS_AUTH_API_TOKEN}
     }
-    @enroll path /api/v1/clients/enroll
+    @enroll path /api/v1/clients/enroll /oidc-callback
     handle @enroll {
         reverse_proxy 192.168.178.182:8090
     }