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/contexts/ToolWorkflowContext.tsx

@@ -4,7 +4,7 @@
  */
 
 import React, { createContext, useContext, useReducer, useCallback, useMemo, useEffect } from 'react';
-import { useToolManagement } from '@app/hooks/useToolManagement';
+import { useToolManagement, type ToolAvailabilityMap } from '@app/hooks/useToolManagement';
 import { PageEditorFunctions } from '@app/types/pageEditor';
 import { ToolRegistryEntry, ToolRegistry } from '@app/data/toolsTaxonomy';
 import { useNavigationActions, useNavigationState } from '@app/contexts/NavigationContext';
@@ -44,6 +44,7 @@ interface ToolWorkflowContextValue extends ToolWorkflowState {
   selectedTool: ToolRegistryEntry | null;
   toolRegistry: Partial<ToolRegistry>;
   getSelectedTool: (toolId: ToolId | null) => ToolRegistryEntry | null;
+  toolAvailability: ToolAvailabilityMap;
 
   // UI Actions
   setSidebarsVisible: (visible: boolean) => void;
@@ -112,7 +113,7 @@ export function ToolWorkflowProvider({ children }: ToolWorkflowProviderProps) {
   const navigationState = useNavigationState();
 
   // Tool management hook
-  const { toolRegistry, getSelectedTool } = useToolManagement();
+  const { toolRegistry, getSelectedTool, toolAvailability } = useToolManagement();
   const { allTools } = useToolRegistry();
 
   // Tool history hook
@@ -258,6 +259,11 @@ export function ToolWorkflowProvider({ children }: ToolWorkflowProviderProps) {
 
   // Workflow actions (compound actions that coordinate multiple state changes)
   const handleToolSelect = useCallback((toolId: ToolId) => {
+    const availabilityInfo = toolAvailability[toolId];
+    const isExplicitlyDisabled = availabilityInfo ? availabilityInfo.available === false : false;
+    if (toolId !== 'read' && toolId !== 'multiTool' && isExplicitlyDisabled) {
+      return;
+    }
     // If we're currently on a custom workbench (e.g., Validate Signature report),
     // selecting any tool should take the user back to the default file manager view.
     const wasInCustomWorkbench = !isBaseWorkbench(navigationState.workbench);
@@ -299,7 +305,7 @@ export function ToolWorkflowProvider({ children }: ToolWorkflowProviderProps) {
     setSearchQuery('');
     setLeftPanelView('toolContent');
     setReaderMode(false); // Disable read mode when selecting tools
-  }, [actions, getSelectedTool, navigationState.workbench, setLeftPanelView, setReaderMode, setSearchQuery]);
+  }, [actions, getSelectedTool, navigationState.workbench, setLeftPanelView, setReaderMode, setSearchQuery, toolAvailability]);
 
   const handleBackToTools = useCallback(() => {
     setLeftPanelView('toolPicker');
@@ -354,6 +360,7 @@ export function ToolWorkflowProvider({ children }: ToolWorkflowProviderProps) {
     toolResetFunctions,
     registerToolReset,
     resetTool,
+    toolAvailability,
 
     // Workflow Actions
     handleToolSelect,
@@ -381,6 +388,7 @@ export function ToolWorkflowProvider({ children }: ToolWorkflowProviderProps) {
     selectedTool,
     toolRegistry,
     getSelectedTool,
+    toolAvailability,
     setSidebarsVisible,
     setLeftPanelView,
     setReaderMode,