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/src/core/services/preferencesService.ts

@@ -9,6 +9,8 @@ export interface UserPreferences {
   toolPanelModePromptSeen: boolean;
   showLegacyToolDescriptions: boolean;
   hasCompletedOnboarding: boolean;
+  hideUnavailableTools: boolean;
+  hideUnavailableConversions: boolean;
 }
 
 export const DEFAULT_PREFERENCES: UserPreferences = {
@@ -19,6 +21,8 @@ export const DEFAULT_PREFERENCES: UserPreferences = {
   toolPanelModePromptSeen: false,
   showLegacyToolDescriptions: false,
   hasCompletedOnboarding: false,
+  hideUnavailableTools: false,
+  hideUnavailableConversions: false,
 };
 
 const STORAGE_KEY = 'stirlingpdf_preferences';

frontend/src/core/services/updateService.ts

@@ -1,6 +1,6 @@
 export interface UpdateSummary {
-  latest_version: string;
-  latest_stable_version?: string;
+  latest_version: string | null;
+  latest_stable_version?: string | null;
   max_priority: 'urgent' | 'normal' | 'minor' | 'low';
   recommended_action?: string;
   any_breaking: boolean;