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/web_test.go

@@ -369,12 +369,6 @@ func TestAuthorizeRequest(t *testing.T) {
 		wantOkNotOverTailscale: false,
 		wantOkWithoutSession:   false,
 		wantOkWithSession:      true,
-	}, {
-		reqPath:                "/api/auth",
-		reqMethod:              httpm.GET,
-		wantOkNotOverTailscale: false,
-		wantOkWithoutSession:   true,
-		wantOkWithSession:      true,
 	}, {
 		reqPath:                "/api/somethingelse",
 		reqMethod:              httpm.GET,
@@ -587,7 +581,7 @@ func TestServeTailscaleAuth(t *testing.T) {
 			r.RemoteAddr = remoteIP
 			r.AddCookie(&http.Cookie{Name: sessionCookieName, Value: tt.cookie})
 			w := httptest.NewRecorder()
-			s.serveTailscaleAuth(w, r)
+			s.serveAPIAuth(w, r)
 			res := w.Result()
 			defer res.Body.Close()