Fix remaining GitHub CodeQL security alerts

- py/stack-trace-exposure: Remove str(e) and traceback.format_exc() from
  all HTTP responses across api_v3.py, pages_v3.py, and app.py; replace
  with generic messages and logger.error(exc_info=True)
- py/reflective-xss: Escape partial_name via markupsafe.escape in the
  load_partial 404 response
- py/path-injection: Add regex validation of plugin_id before filesystem
  use in _load_plugin_config_partial
- py/incomplete-url-substring-sanitization: Replace 'github.com' in
  substring checks with urlparse hostname comparison in store_manager.py
- py/clear-text-logging-sensitive-data: Remove football-scoreboard debug
  prints and sensitive request-body prints from update endpoint
- js/bad-tag-filter: Replace script-only regex in BaseWidget.sanitizeValue
  with DOM-based textContent stripping that removes all HTML
- js/incomplete-sanitization: Fix escapeAttr to properly encode &, ", ',
  <, > using HTML entities instead of backslash escaping
- js/prototype-pollution-utility: Add __proto__/constructor/prototype
  key guards to deepMerge function in plugins_manager.js
- app.py error handlers: Always return generic messages; remove debug-mode
  branches that could expose tracebacks in production

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web_interface/static/v3/plugins_manager.js

@@ -1442,9 +1442,14 @@ function renderInstalledPlugins(plugins) {
         return;
     }
 
-    // Helper function to escape attributes for use in HTML
+    // Helper function to escape values for use in HTML attributes
     const escapeAttr = (text) => {
-        return (text || '').replace(/'/g, "\\'").replace(/"/g, '"');
+        return (text || '')
+            .replace(/&/g, '&')
+            .replace(/"/g, '"')
+            .replace(/'/g, ''')
+            .replace(/</g, '&lt;')
+            .replace(/>/g, '&gt;');
     };
 
     // Helper function to escape for JavaScript strings (use JSON.stringify for proper escaping)
@@ -4507,6 +4512,8 @@ function syncFormToJson() {
     // Deep merge with existing config to preserve nested structures
     function deepMerge(target, source) {
         for (const key in source) {
+            if (key === '__proto__' || key === 'constructor' || key === 'prototype') continue;
+            if (!Object.prototype.hasOwnProperty.call(source, key)) continue;
             if (source[key] && typeof source[key] === 'object' && !Array.isArray(source[key])) {
                 if (!target[key] || typeof target[key] !== 'object' || Array.isArray(target[key])) {
                     target[key] = {};