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'

cmd/desktop/main.go

@@ -23,6 +23,9 @@ import (
 //go:embed frontend/dist
 var assets embed.FS
 
+//go:embed icon.png
+var iconPNG []byte
+
 const (
 	keyringService = "com.hubris.oikos-desktop"
 	keyringUser    = "oikos"
@@ -322,10 +325,12 @@ func main() {
 	// --- System tray ---
 	systemTray := app.SystemTray.New()
 	systemTray.SetLabel("Oikos")
-	systemTray.SetTooltip("Oikos — Control Room")
+	systemTray.SetTooltip("Oikos")
+	systemTray.SetIcon(iconPNG)
+	systemTray.SetTemplateIcon(iconPNG)
 
 	trayMenu := application.NewMenu()
-	trayMenu.Add("Open Control Room").OnClick(func(ctx *application.Context) {
+	trayMenu.Add("Open Oikos").OnClick(func(ctx *application.Context) {
 		for _, w := range app.Window.GetAll() {
 			w.Show()
 			w.Focus()
@@ -347,7 +352,7 @@ func main() {
 	minWidth, minHeight := 1024, 700
 
 	window := app.Window.NewWithOptions(application.WebviewWindowOptions{
-		Title:    "Oikos — Control Room",
+		Title:    "Oikos",
 		Width:    width,
 		Height:   height,
 		MinWidth: minWidth,