feat(web): Phase 6.5c implement [Y/n] execution wiring

DualStateIncidentCard:
- Add proposalId prop for approval actions
- Add onApprovalChange callback for status updates
- Implement handleApprove() calling POST /api/v1/approvals/{id}/sign
- Implement handleReject() calling POST /api/v1/approvals/{id}/reject
- Add ButtonState management (idle/loading/approved/rejected/error)
- Loading spinner during API call
- Success state: green "已授權" / red "已拒絕"
- Error state: orange "錯誤" with auto-recovery

API Client:
- Fix endpoint mismatch: rename approveApproval to signApproval
- Use correct endpoint /sign instead of /approve
- Add signer parameter for multi-sig support

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/src/lib/api-client.ts

@@ -114,13 +114,19 @@ export const apiClient = {
     }>(res)
   },
 
-  async approveApproval(approvalId: string, reason?: string) {
-    const res = await fetch(`${API_BASE_URL}/approvals/${approvalId}/approve`, {
+  async signApproval(approvalId: string, signer: string = 'commander', reason?: string) {
+    const res = await fetch(`${API_BASE_URL}/approvals/${approvalId}/sign`, {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
-      body: JSON.stringify({ reason }),
+      body: JSON.stringify({ signer, reason }),
     })
-    return handleResponse<{ id: string; status: string }>(res)
+    return handleResponse<{
+      approval_id: string
+      status: string
+      current_signatures: number
+      required_signatures: number
+      message: string
+    }>(res)
   },
 
   async rejectApproval(approvalId: string, reason?: string) {