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/tenant/TeamPage.tsx

@@ -1,4 +1,5 @@
 import { useState } from 'react';
+import { errorMessage } from '../../api/client';
 import {
   Alert,
   AlertDialog,
@@ -131,7 +132,7 @@ export function TeamPage() {
       setInviteRole('viewer');
       setShowInvite(false);
     } catch (err) {
-      toast({ title: 'Invite failed', description: String(err), variant: 'error' });
+      toast({ title: 'Invite failed', description: errorMessage(err), variant: 'error' });
     }
   }
 
@@ -142,7 +143,7 @@ export function TeamPage() {
       toast({ title: `Removed ${removeTarget.name}`, variant: 'success' });
       setRemoveTarget(null);
     } catch (err) {
-      toast({ title: 'Remove failed', description: String(err), variant: 'error' });
+      toast({ title: 'Remove failed', description: errorMessage(err), variant: 'error' });
       setRemoveTarget(null);
     }
   }
@@ -269,7 +270,7 @@ export function TeamPage() {
                   toast({ title: `MFA reset for ${mfaResetTarget.name || mfaResetTarget.email}`, variant: 'success' });
                   setMfaResetTarget(null);
                 } catch (err) {
-                  toast({ title: 'Failed to reset MFA', description: String(err), variant: 'error' });
+                  toast({ title: 'Failed to reset MFA', description: errorMessage(err), variant: 'error' });
                 }
               }}
               loading={resetMfa.isPending}
@@ -297,7 +298,7 @@ export function TeamPage() {
                 setPwTarget(null);
                 setPwValue('');
               } catch (err) {
-                toast({ title: 'Reset failed', description: String(err), variant: 'error' });
+                toast({ title: 'Reset failed', description: errorMessage(err), variant: 'error' });
               }
             }}
             style={{ display: 'flex', flexDirection: 'column', gap: 16 }}