feat(auth): block panel with default admin/admin credentials and guide credential change

checkLogin middleware now detects default admin/admin credentials and
redirects every panel route to /panel/settings until they are changed.
The settings page auto-opens the Authentication tab, shows a
non-dismissible error banner, and lists 'Default credentials' first in
the security checklist. Login response includes mustChangeCredentials
so the login page can redirect directly. Logout is now POST-only.
Password must be at least 10 characters and cannot be admin/admin.

web/controller/dist.go

@@ -57,11 +57,18 @@ func serveDistPage(c *gin.Context, name string) {
 	}
 	csrfMeta := []byte(`<meta name="csrf-token" content="` + htmlpkg.EscapeString(csrfToken) + `">`)
 
-	script := `<script>window.X_UI_BASE_PATH="` + escapedBase + `"`
+	nonceAttr := ""
+	if nonce := c.GetString("csp_nonce"); nonce != "" {
+		nonceAttr = ` nonce="` + htmlpkg.EscapeString(nonce) + `"`
+	}
+	script := `<script` + nonceAttr + `>window.X_UI_BASE_PATH="` + escapedBase + `"`
 	if name != "login.html" {
 		escapedVer := jsEscape.Replace(config.GetVersion())
 		script += `;window.X_UI_CUR_VER="` + escapedVer + `"`
 	}
+	if u := session.GetLoginUser(c); u != nil && isDefaultAdminCredential(u.Username, u.Password) {
+		script += `;window.X_UI_MUST_CHANGE_CREDENTIALS=true`
+	}
 	script += `;</script>`
 	inject := []byte(script)
 	inject = append(inject, csrfMeta...)