client/web: limit authorization checks to API calls

This completes the migration to setting up authentication state in the
client first before fetching any node data or rendering the client view.

Notable changes:
 - `authorizeRequest` is now only enforced on `/api/*` calls (with the
   exception of /api/auth, which is handled early because it's needed to
   initially setup auth, particularly for synology)
 - re-separate the App and WebClient components to ensure that auth is
   completed before moving on
 - refactor platform auth (synology and QNAP) to fit into this new
   structure. Synology no longer returns redirect for auth, but returns
   authResponse instructing the client to fetch a SynoToken

Updates tailscale/corp#14335

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

client/web/qnap.go

@@ -9,6 +9,7 @@ package web
 import (
 	"crypto/tls"
 	"encoding/xml"
+	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -18,21 +19,17 @@ import (
 
 // authorizeQNAP authenticates the logged-in QNAP user and verifies that they
 // are authorized to use the web client.
-// It reports true if the request is authorized to continue, and false otherwise.
-// authorizeQNAP manages writing out any relevant authorization errors to the
-// ResponseWriter itself.
-func authorizeQNAP(w http.ResponseWriter, r *http.Request) (ok bool) {
+// If the user is not authorized to use the client, an error is returned.
+func authorizeQNAP(r *http.Request) (ar authResponse, err error) {
 	_, resp, err := qnapAuthn(r)
 	if err != nil {
-		http.Error(w, err.Error(), http.StatusUnauthorized)
-		return false
+		return ar, err
 	}
 	if resp.IsAdmin == 0 {
-		http.Error(w, "user is not an admin", http.StatusForbidden)
-		return false
+		return ar, errors.New("user is not an admin")
 	}
 
-	return true
+	return authResponse{OK: true}, nil
 }
 
 type qnapAuthResponse struct {