feat(server): add ExtAuth logout URL configuration (#5074)

* feat(server): add ExtAuth logout URL configuration (#4467)

When external authentication (reverse proxy auth) is active, the Logout
button is hidden because authentication is managed externally. Many
external auth services (Authelia, Authentik, Keycloak) provide a logout
URL that can terminate the session.

Add `ExtAuth.LogoutURL` config option that, when set, shows the Logout
button in the UI and redirects the user to the external auth provider's
logout endpoint instead of the Navidrome login page.

* feat(server): add validation for ExtAuth logout URL configuration

* feat(server): refactor ExtAuth logout URL validation to a reusable function

* fix(configuration): rename URL validation functions for consistency

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(configuration): rename URL validation functions for consistency

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

conf/configuration_test.go

@@ -52,6 +52,48 @@ var _ = Describe("Configuration", func() {
 		})
 	})
 
+	Describe("ValidateURL", func() {
+		It("accepts a valid http URL", func() {
+			fn := conf.ValidateURL("TestOption", "http://example.com/path")
+			Expect(fn()).To(Succeed())
+		})
+
+		It("accepts a valid https URL", func() {
+			fn := conf.ValidateURL("TestOption", "https://example.com/path")
+			Expect(fn()).To(Succeed())
+		})
+
+		It("rejects a URL with no scheme", func() {
+			fn := conf.ValidateURL("TestOption", "example.com/path")
+			Expect(fn()).To(MatchError(ContainSubstring("invalid scheme")))
+		})
+
+		It("rejects a URL with an unsupported scheme", func() {
+			fn := conf.ValidateURL("TestOption", "javascript://example.com/path")
+			Expect(fn()).To(MatchError(ContainSubstring("invalid scheme")))
+		})
+
+		It("accepts an empty URL (optional config)", func() {
+			fn := conf.ValidateURL("TestOption", "")
+			Expect(fn()).To(Succeed())
+		})
+
+		It("includes the option name in the error message", func() {
+			fn := conf.ValidateURL("MyOption", "ftp://example.com")
+			Expect(fn()).To(MatchError(ContainSubstring("MyOption")))
+		})
+
+		It("rejects a URL that cannot be parsed", func() {
+			fn := conf.ValidateURL("TestOption", "://invalid")
+			Expect(fn()).To(HaveOccurred())
+		})
+
+		It("rejects a URL without a host", func() {
+			fn := conf.ValidateURL("TestOption", "http:///path")
+			Expect(fn()).To(MatchError(ContainSubstring("non-empty host is required")))
+		})
+	})
+
 	DescribeTable("NormalizeSearchBackend",
 		func(input, expected string) {
 			Expect(conf.NormalizeSearchBackend(input)).To(Equal(expected))