fix(auth): invalidate sessions when 2FA is enabled, fix dev 401 loop

Add UserService.BumpLoginEpoch and call it from updateSetting when
TwoFactorEnable flips false → true. Existing cookies (issued under
the looser no-2FA policy) get a 401 on their next request and are
forced through the login flow. Disabling 2FA is a relaxation and
does not bump the epoch — sessions stay valid.

Also fix the dev-mode 401 redirect: targeting `${basePath}login.html`
breaks when basePath isn't "/" (Vite has no file at e.g.
"/test/login.html"; the SPA fallback loops the 401). Navigate to
basePath instead — Vite's bypassMigratedRoute and Go's index
handler both serve login.html for that path.

Strip stale doc-comment from netsafe and IndexController.logout
in line with the project's no-inline-comments convention.

web/service/user.go

@@ -102,6 +102,14 @@ func (s *UserService) CheckUser(username string, password string, twoFactorCode
 	return user, nil
 }
 
+func (s *UserService) BumpLoginEpoch() error {
+	db := database.GetDB()
+	return db.Model(model.User{}).
+		Where("1 = 1").
+		Update("login_epoch", gorm.Expr("login_epoch + 1")).
+		Error
+}
+
 func (s *UserService) UpdateUser(id int, username string, password string) error {
 	db := database.GetDB()
 	hashedPassword, err := crypto.HashPasswordAsBcrypt(password)