client/web: add new readonly mode

The new read-only mode is only accessible when running `tailscale web`
by passing a new `-readonly` flag. This new mode is identical to the
existing login mode with two exceptions:

 - the management client in tailscaled is not started (though if it is
   already running, it is left alone)

 - the client does not prompt the user to login or switch to the
   management client. Instead, a message is shown instructing the user
   to use other means to manage the device.

Updates #10979

Signed-off-by: Will Norris <will@tailscale.com>

client/web/web.go

@@ -95,6 +95,14 @@ const (
 	// In this mode, API calls are authenticated via platform auth.
 	LoginServerMode ServerMode = "login"
 
+	// ReadOnlyServerMode is identical to LoginServerMode,
+	// but does not present a login button to switch to manage mode,
+	// even if the management client is running and reachable.
+	//
+	// This is designed for platforms where the device is configured by other means,
+	// such as Home Assistant's declarative YAML configuration.
+	ReadOnlyServerMode ServerMode = "readonly"
+
 	// ManageServerMode serves a management client for editing tailscale
 	// settings of a node.
 	//
@@ -154,7 +162,7 @@ type ServerOpts struct {
 // and not the lifespan of the web server.
 func NewServer(opts ServerOpts) (s *Server, err error) {
 	switch opts.Mode {
-	case LoginServerMode, ManageServerMode:
+	case LoginServerMode, ReadOnlyServerMode, ManageServerMode:
 		// valid types
 	case "":
 		return nil, fmt.Errorf("must specify a Mode")
@@ -207,10 +215,14 @@ func NewServer(opts ServerOpts) (s *Server, err error) {
 	// The client is secured by limiting the interface it listens on,
 	// or by authenticating requests before they reach the web client.
 	csrfProtect := csrf.Protect(s.csrfKey(), csrf.Secure(false))
-	if s.mode == LoginServerMode {
+	switch s.mode {
+	case LoginServerMode:
 		s.apiHandler = csrfProtect(http.HandlerFunc(s.serveLoginAPI))
 		metric = "web_login_client_initialization"
-	} else {
+	case ReadOnlyServerMode:
+		s.apiHandler = csrfProtect(http.HandlerFunc(s.serveLoginAPI))
+		metric = "web_readonly_client_initialization"
+	case ManageServerMode:
 		s.apiHandler = csrfProtect(http.HandlerFunc(s.serveAPI))
 		metric = "web_client_initialization"
 	}
@@ -483,7 +495,7 @@ func (s *Server) serveAPIAuth(w http.ResponseWriter, r *http.Request) {
 
 	// First verify platform auth.
 	// If platform auth is needed, this should happen first.
-	if s.mode == LoginServerMode {
+	if s.mode == LoginServerMode || s.mode == ReadOnlyServerMode {
 		switch distro.Get() {
 		case distro.Synology:
 			authorized, err := authorizeSynology(r)