feat: add vendor notifications and analytics pages (Phase 3)

New pages:
- Notifications center: view, mark read, delete, settings modal
- Analytics: period selector, stats overview, feature-gated advanced metrics

Changes:
- Add routes for /vendor/{code}/notifications and /analytics
- Create notifications.js and analytics.js with full functionality
- Create notifications.html and analytics.html templates
- Update sidebar with Analytics link and Notifications in Customers section
- Update vendor-frontend-parity-plan.md to mark Phase 3 complete

Vendor frontend now at ~95% parity with admin.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/routes/vendor_pages.py

@@ -527,6 +527,60 @@ async def vendor_billing_page(
     )
 
 
+# ============================================================================
+# NOTIFICATIONS
+# ============================================================================
+
+
+@router.get(
+    "/{vendor_code}/notifications", response_class=HTMLResponse, include_in_schema=False
+)
+async def vendor_notifications_page(
+    request: Request,
+    vendor_code: str = Path(..., description="Vendor code"),
+    current_user: User = Depends(get_current_vendor_from_cookie_or_header),
+):
+    """
+    Render notifications center page.
+    JavaScript loads notifications via API.
+    """
+    return templates.TemplateResponse(
+        "vendor/notifications.html",
+        {
+            "request": request,
+            "user": current_user,
+            "vendor_code": vendor_code,
+        },
+    )
+
+
+# ============================================================================
+# ANALYTICS
+# ============================================================================
+
+
+@router.get(
+    "/{vendor_code}/analytics", response_class=HTMLResponse, include_in_schema=False
+)
+async def vendor_analytics_page(
+    request: Request,
+    vendor_code: str = Path(..., description="Vendor code"),
+    current_user: User = Depends(get_current_vendor_from_cookie_or_header),
+):
+    """
+    Render analytics and reports page.
+    JavaScript loads analytics data via API.
+    """
+    return templates.TemplateResponse(
+        "vendor/analytics.html",
+        {
+            "request": request,
+            "user": current_user,
+            "vendor_code": vendor_code,
+        },
+    )
+
+
 # ============================================================================
 # CONTENT PAGES MANAGEMENT
 # ============================================================================