fix(ui): extract meaningful error messages from API responses

Introduces ApiError class in client.ts that parses Spring Boot error
bodies to extract human-readable messages (message, error, detail fields).
Adds errorMessage() helper used by all toast descriptions instead of
raw String(err) which dumped JSON blobs to the user.

Affected: all 10 page components that display error toasts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/vendor/CertificatesPage.tsx

@@ -1,4 +1,5 @@
 import { useRef, useState } from 'react';
+import { errorMessage } from '../../api/client';
 import {
   Alert,
   Badge,
@@ -163,7 +164,7 @@ export function CertificatesPage() {
         toast({ title: 'Validation failed', description: result.errors.join(', '), variant: 'error' });
       }
     } catch (err) {
-      toast({ title: 'Upload failed', description: String(err), variant: 'error' });
+      toast({ title: 'Upload failed', description: errorMessage(err), variant: 'error' });
     }
   }
 
@@ -172,7 +173,7 @@ export function CertificatesPage() {
       await activateMutation.mutateAsync();
       toast({ title: 'Certificate activated', variant: 'success' });
     } catch (err) {
-      toast({ title: 'Activation failed', description: String(err), variant: 'error' });
+      toast({ title: 'Activation failed', description: errorMessage(err), variant: 'error' });
     }
   }
 
@@ -181,7 +182,7 @@ export function CertificatesPage() {
       await restoreMutation.mutateAsync();
       toast({ title: 'Certificate restored from archive', variant: 'success' });
     } catch (err) {
-      toast({ title: 'Restore failed', description: String(err), variant: 'error' });
+      toast({ title: 'Restore failed', description: errorMessage(err), variant: 'error' });
     }
   }
 
@@ -190,7 +191,7 @@ export function CertificatesPage() {
       await discardMutation.mutateAsync();
       toast({ title: 'Staged certificate discarded', variant: 'success' });
     } catch (err) {
-      toast({ title: 'Discard failed', description: String(err), variant: 'error' });
+      toast({ title: 'Discard failed', description: errorMessage(err), variant: 'error' });
     }
   }