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/pages_v3.py

@@ -1,4 +1,5 @@
 from flask import Blueprint, render_template, flash
+from jinja2 import TemplateNotFound
 from markupsafe import escape
 import json
 import logging
@@ -454,9 +455,12 @@ def _load_tools_partial():
     """Load tools/utilities partial."""
     try:
         return render_template('v3/partials/tools.html')
-    except Exception:
-        logger.error("Error loading partial", exc_info=True)
-        return "Error loading partial", 500
+    except TemplateNotFound:
+        logger.error("[Pages V3][Tools] Template not found: v3/partials/tools.html", exc_info=True)
+        return "[Pages V3][Tools] Template is missing.", 500
+    except OSError as exc:
+        logger.error("[Pages V3][Tools] I/O error loading tools partial: %s", exc, exc_info=True)
+        return "[Pages V3][Tools] Failed to load due to a file system error. Check logs.", 500
 
 
 def _load_plugin_config_partial(plugin_id):