feat(1099): add reveal-full-TIN button (write-gated) to recipient dialog

Wires the existing GET payee-1099/{id}/tin endpoint into the edit dialog:
a "Reveal full TIN" button guarded by *appHasPermission Form1099:write that
fetches and displays the decrypted TIN read-only. Satisfies acceptance
criterion #11.4. The value is never logged or persisted.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

APP/src/app/features/payee1099/pages/payee-1099-page/payee-1099-page.component.html

@@ -103,6 +103,12 @@
         <kendo-textbox [(ngModel)]="form.tin"
           [placeholder]="editingId != null && editingTinLast4 ? '***-**-' + editingTinLast4 : ''"></kendo-textbox>
         <span *ngIf="editingId != null" class="hint-text-sm">Leave blank to keep the existing TIN / 留空則保留現有稅號</span>
+        <div *ngIf="editingId != null" class="flex flex-col gap-1">
+          <button kendoButton type="button" fillMode="link" class="self-start"
+            *appHasPermission="{ module: 'Form1099', action: 'write' }"
+            (click)="revealTin()">Reveal full TIN / 顯示完整 TIN</button>
+          <span *ngIf="revealedTin" class="font-mono">{{ revealedTin }}</span>
+        </div>
       </label>
 
       <label class="flex flex-col gap-1 md:col-span-2">