fix: inquiry edit form - make customer fields read-only

P1-04: Inquiry 수정 계약 확정 - 화면과 저장 필드 불일치 제거

Problem: InquiryEdit showed editable fields for Name/Phone/Email/Message, but
UpdateInquiryDto only saved Status/AdminMemo. Users could edit fields that had
no effect on save - the 'false affordance' anti-pattern.

Solution:
- Add IsEditMode parameter to InquiryForm
- When IsEditMode=true: bind Name/Phone/Email/Message as ReadOnly (disabled input)
- Update InquiryEdit to pass IsEditMode="true"
- InquiryCreate passes default false, keeping all fields editable

Result: Edit mode now clearly shows which fields are modifiable (Status, AdminMemo)
vs. informational (customer contact details, message text). UI matches API contract.

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

src/TaxBaik.Web.Client/Components/Admin/Pages/Inquiries/InquiryEdit.razor

@@ -23,7 +23,7 @@
 else
 {
     <MudPaper Class="pa-4 mt-4" Elevation="1">
-        <InquiryForm ButtonText="수정" InitialData="formModel" OnSubmit="HandleUpdate" OnCancel="GoBack" />
+        <InquiryForm ButtonText="수정" InitialData="formModel" OnSubmit="HandleUpdate" OnCancel="GoBack" IsEditMode="true" />
 
         <MudDivider Class="my-4" />