Differentiate unavailable tools by reason (#4916)

## Summary
- track endpoint disable reasons server-side and expose them through a
new `/api/v1/config/endpoints-availability` API that the frontend can
consume
- refresh the web UI tool management logic to cache endpoint details,
compute per-tool availability metadata, and show reason-specific
messaging (admin disabled vs missing dependency) when a tool cannot be
launched
- add the missing en-GB translations for the new unavailability labels
so the UI copy reflects the new distinction
<img width="1156" height="152" alt="image"
src="https://github.com/user-attachments/assets/b54eda37-fe5c-42f9-bd5f-9ee00398d1ae"
/>


<img width="930" height="168" alt="image"
src="https://github.com/user-attachments/assets/47c07ffa-adb7-4ce3-910c-b6ff73f6f993"
/>


## Testing
- `npm run typecheck:core` *(fails:
frontend/src/core/components/shared/LocalIcon.tsx expects
../../../assets/material-symbols-icons.json, which is not present in
this environment)*

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_b_6919af7a493c8328bb5ac3d07e65452b)

frontend/public/locales/en-GB/translation.json

@@ -17,6 +17,8 @@
       "comingSoon": "Coming soon:",
       "favorite": "Add to favourites",
       "favorites": "Favourites",
+      "unavailable": "Disabled by server administrator:",
+      "unavailableDependency": "Unavailable - required tool missing on server:",
       "heading": "All tools (fullscreen view)",
       "noResults": "Try adjusting your search or toggle descriptions to find what you need.",
       "recommended": "Recommended",
@@ -5721,7 +5723,11 @@
         "latestVersion": "Latest Version",
         "checkForUpdates": "Check for Updates",
         "viewDetails": "View Details"
-      }
+      },
+      "hideUnavailableTools": "Hide unavailable tools",
+      "hideUnavailableToolsDescription": "Remove tools that have been disabled by your server instead of showing them greyed out.",
+      "hideUnavailableConversions": "Hide unavailable conversions",
+      "hideUnavailableConversionsDescription": "Remove disabled conversion options in the Convert tool instead of showing them greyed out."
     },
     "hotkeys": {
       "errorConflict": "Shortcut already used by {{tool}}.",