feat: add vendor messaging interface

- Add vendor messages API endpoints (/api/v1/vendor/messages)
- Add vendor messages page route (/vendor/{code}/messages)
- Add messages.html template for vendor portal
- Add messages.js Alpine component
- Add Messages link to vendor sidebar under Sales section

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

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

app/routes/vendor_pages.py

@@ -219,6 +219,59 @@ async def vendor_customers_page(
     )
 
 
+# ============================================================================
+# MESSAGING
+# ============================================================================
+
+
+@router.get(
+    "/{vendor_code}/messages", response_class=HTMLResponse, include_in_schema=False
+)
+async def vendor_messages_page(
+    request: Request,
+    vendor_code: str = Path(..., description="Vendor code"),
+    current_user: User = Depends(get_current_vendor_from_cookie_or_header),
+):
+    """
+    Render messages page.
+    JavaScript loads conversations and messages via API.
+    """
+    return templates.TemplateResponse(
+        "vendor/messages.html",
+        {
+            "request": request,
+            "user": current_user,
+            "vendor_code": vendor_code,
+        },
+    )
+
+
+@router.get(
+    "/{vendor_code}/messages/{conversation_id}",
+    response_class=HTMLResponse,
+    include_in_schema=False,
+)
+async def vendor_message_detail_page(
+    request: Request,
+    vendor_code: str = Path(..., description="Vendor code"),
+    conversation_id: int = Path(..., description="Conversation ID"),
+    current_user: User = Depends(get_current_vendor_from_cookie_or_header),
+):
+    """
+    Render message detail page.
+    Shows the full conversation thread.
+    """
+    return templates.TemplateResponse(
+        "vendor/messages.html",
+        {
+            "request": request,
+            "user": current_user,
+            "vendor_code": vendor_code,
+            "conversation_id": conversation_id,
+        },
+    )
+
+
 # ============================================================================
 # INVENTORY MANAGEMENT
 # ============================================================================