fix(tools-tab): resolve remaining PR review comments

- api_v3: use getattr(api_v3, 'plugin_manager', None) instead of the
  module-level plugin_manager (always None); app.py sets the blueprint
  attribute, not the module global, so the fallback to plugin-repos was
  always taken
- pages_v3: replace broad except Exception in _load_tools_partial with
  specific TemplateNotFound / OSError handlers and add [Pages V3][Tools]
  context prefix to log messages and error responses for easier Pi
  debugging
- base.html: add Tools tab branch to the HTMX-unavailable fallback block
  in loadTabContent so the tab loads gracefully via direct fetch if HTMX
  never initialises

Skipped: auth on execute_system_action — pre-existing app-wide design;
reboot/shutdown and all other system actions share the same exposure.
An app-level auth layer is the correct fix and is out of scope here.

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

web_interface/blueprints/api_v3.py

@@ -1666,7 +1666,8 @@ def execute_system_action():
                 'output': _truncate_output(result.stdout, result.stderr)
             })
         elif action == 'install_plugin_requirements':
-            plugins_dir = Path(plugin_manager.plugins_dir) if plugin_manager else PROJECT_ROOT / 'plugin-repos'
+            active_pm = getattr(api_v3, 'plugin_manager', None)
+            plugins_dir = Path(active_pm.plugins_dir) if active_pm else PROJECT_ROOT / 'plugin-repos'
             results = []
             if plugins_dir.exists():
                 for p in sorted(plugins_dir.iterdir()):