feat: add CRM & Tax Management navigation group in admin sidebar

Step 4: Navigation Reorganization
- Add new 'CRM & 세무관리' nav group (BusinessCenter icon)
- Organize 5 CRM pages: TaxProfiles, TaxFilingSchedules, Contracts, ConsultingActivities, RevenueTrackings
- Reorder nav groups: Dashboard → CRM (default expanded) → Customer → Website → Inquiries → Settings
- Update 'Customer' group label and icons for clarity
- Set expandedCRMGroup=true for immediate visibility

Navigation Structure (Post-change):
`
대시보드
├─ CRM & 세무관리 [EXPANDED]
│  ├─ 세무 프로필 (Assignment icon)
│  ├─ 신고 일정 (CalendarMonth icon)
│  ├─ 계약 관리 (Description icon)
│  ├─ 상담 활동 (ChatBubble icon)
│  └─ 수익 추적 (Receipt icon)
├─ 고객 관리
│  ├─ 고객 카드
│  └─ 세무신고
├─ 홈페이지
│  ├─ 공지사항
│  ├─ FAQ 관리
│  ├─ 블로그 관리
│  └─ 시즌 시뮬레이터
├─ 문의 관리
└─ 설정
`

Design Rationale:
- CRM group positioned first (after dashboard) for workflow priority
- Default expanded = immediate page discovery
- Icons from Material Design Filled set for consistency
- Grouped by business domain, not by data type

Build Status: 0 errors, 3 warnings (existing)

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

TaxBaik.Web/Components/Admin/Layout/MainLayout.razor

@@ -59,16 +59,27 @@
         </div>
         <MudNavMenu Class="admin-nav">
             <MudNavLink Href="/taxbaik/admin/dashboard" Match="NavLinkMatch.All" Icon="@Icons.Material.Filled.Dashboard">대시보드</MudNavLink>
+
+            <MudNavGroup Title="CRM & 세무관리" Icon="@Icons.Material.Filled.BusinessCenter" @bind-Expanded="@expandedCRMGroup">
+                <MudNavLink Href="/taxbaik/admin/tax-profiles" Icon="@Icons.Material.Filled.Assignment">세무 프로필</MudNavLink>
+                <MudNavLink Href="/taxbaik/admin/tax-filing-schedules" Icon="@Icons.Material.Filled.CalendarMonth">신고 일정</MudNavLink>
+                <MudNavLink Href="/taxbaik/admin/contracts" Icon="@Icons.Material.Filled.Description">계약 관리</MudNavLink>
+                <MudNavLink Href="/taxbaik/admin/consulting-activities" Icon="@Icons.Material.Filled.ChatBubble">상담 활동</MudNavLink>
+                <MudNavLink Href="/taxbaik/admin/revenue-trackings" Icon="@Icons.Material.Filled.Receipt">수익 추적</MudNavLink>
+            </MudNavGroup>
+
             <MudNavGroup Title="고객 관리" Icon="@Icons.Material.Filled.PeopleAlt" @bind-Expanded="@expandedCustomerGroup">
                 <MudNavLink Href="/taxbaik/admin/clients" Icon="@Icons.Material.Filled.ContactPage">고객 카드</MudNavLink>
-                <MudNavLink Href="/taxbaik/admin/tax-filings" Icon="@Icons.Material.Filled.CalendarMonth">신고 일정</MudNavLink>
+                <MudNavLink Href="/taxbaik/admin/tax-filings" Icon="@Icons.Material.Filled.Assessment">세무신고</MudNavLink>
             </MudNavGroup>
+
             <MudNavGroup Title="홈페이지" Icon="@Icons.Material.Filled.Home" @bind-Expanded="@expandedWebsiteGroup">
                 <MudNavLink Href="/taxbaik/admin/announcements" Icon="@Icons.Material.Filled.Campaign">공지사항</MudNavLink>
                 <MudNavLink Href="/taxbaik/admin/faqs" Icon="@Icons.Material.Filled.QuestionAnswer">FAQ 관리</MudNavLink>
                 <MudNavLink Href="/taxbaik/admin/blog" Icon="@Icons.Material.Filled.Article">블로그 관리</MudNavLink>
                 <MudNavLink Href="/taxbaik/admin/season-simulator" Icon="@Icons.Material.Filled.Preview">시즌 시뮬레이터</MudNavLink>
             </MudNavGroup>
+
             <MudNavLink Href="/taxbaik/admin/inquiries" Icon="@Icons.Material.Filled.Forum">문의 관리</MudNavLink>
             <MudNavLink Href="/taxbaik/admin/settings" Icon="@Icons.Material.Filled.Tune">설정</MudNavLink>
         </MudNavMenu>
@@ -101,7 +112,8 @@
 
 @code {
     private bool drawerOpen = true;
-    private bool expandedCustomerGroup = true;
+    private bool expandedCRMGroup = true;
+    private bool expandedCustomerGroup = false;
     private bool expandedWebsiteGroup = false;
 
     protected override void OnInitialized()