templates: generalise auth templates for web and OIDC

Extract shared HTML/CSS design into a common template and create
generalised auth success and web auth templates that work for both
node registration and SSH check authentication flows.

Updates #1850

hscontrol/templates/auth_success.go

@@ -0,0 +1,62 @@
+package templates
+
+import (
+	"github.com/chasefleming/elem-go"
+)
+
+// AuthSuccessResult contains the text content for an authentication success page.
+// Each field controls a distinct piece of user-facing text so that every auth
+// flow (node registration, reauthentication, SSH check, …) can clearly
+// communicate what just happened.
+type AuthSuccessResult struct {
+	// Title is the browser tab / page title,
+	// e.g. "Headscale - Node Registered".
+	Title string
+
+	// Heading is the bold green text inside the success box,
+	// e.g. "Node registered".
+	Heading string
+
+	// Verb is the action prefix in the body text before "as <user>",
+	// e.g. "Registered", "Reauthenticated", "Authorized".
+	Verb string
+
+	// User is the display name shown in bold in the body text,
+	// e.g. "user@example.com".
+	User string
+
+	// Message is the follow-up instruction shown after the user name,
+	// e.g. "You can now close this window."
+	Message string
+}
+
+// AuthSuccess renders an authentication / authorisation success page.
+// The caller controls every user-visible string via [AuthSuccessResult] so the
+// page clearly describes what succeeded (registration, reauth, SSH check, …).
+func AuthSuccess(result AuthSuccessResult) *elem.Element {
+	box := successBox(
+		result.Heading,
+		elem.Text(result.Verb+" as "),
+		elem.Strong(nil, elem.Text(result.User)),
+		elem.Text(". "+result.Message),
+	)
+
+	return HtmlStructure(
+		elem.Title(nil, elem.Text(result.Title)),
+		mdTypesetBody(
+			headscaleLogo(),
+			box,
+			H2(elem.Text("Getting started")),
+			P(elem.Text("Check out the documentation to learn more about headscale and Tailscale:")),
+			Ul(
+				elem.Li(nil,
+					externalLink("https://headscale.net/stable/", "Headscale documentation"),
+				),
+				elem.Li(nil,
+					externalLink("https://tailscale.com/kb/", "Tailscale knowledge base"),
+				),
+			),
+			pageFooter(),
+		),
+	)
+}