fix: skip backup dirs in plugin discovery and fix HTMX event syntax

- plugin_manager.py: skip directories containing '.standalone-backup-'
  during discovery scan, matching state_reconciliation.py behavior and
  preventing backup manifests from overwriting live plugin entries
- base.html: fix hx-on::htmx:response-error → hx-on::response-error
  (the :: shorthand already adds the htmx: prefix, so the original
  syntax resolved to htmx:htmx:response-error making the handler dead)

Skipped findings:
- web-ui-info in _SYSTEM_CONFIG_KEYS: it's a real plugin with manifest.json
  and config entry, not a system key
- store_manager config key migration: valid feature request for handling
  ledmatrix- prefix rename, but new functionality outside this PR scope

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/plugin_system/plugin_manager.py

@@ -119,6 +119,9 @@ class PluginManager:
             for item in directory.iterdir():
                 if not item.is_dir():
                     continue
+                # Skip backup directories so they don't overwrite live entries
+                if '.standalone-backup-' in item.name:
+                    continue
 
                 manifest_path = item / "manifest.json"
                 if manifest_path.exists():