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/controller/index.go

@@ -135,7 +135,6 @@ func loginFailureReason(err error) string {
 	return "invalid credentials"
 }
 
-// logout clears the session. The SPA performs the navigation client-side.
 func (a *IndexController) logout(c *gin.Context) {
 	user := session.GetLoginUser(c)
 	if user != nil {