feat(dashboard): redesign UI with smart polling, unified sync card, filter bar

Replace SSE with smart polling (30s idle / 3s when running). Unify sync
panel into single two-row card with live progress text. Add unified filter
bar (period dropdown, status pills, search) with period-total counts.
Add Client/Cont tooltip for different shipping/billing persons. Add SKU
mappings pct_total badges + complete/incomplete filter + 409 duplicate
check. Add missing SKUs search + rescan progress UX. Migrate SQLite
orders schema (shipping_name, billing_name, payment_method,
delivery_method). Fix JSON_OUTPUT_DIR path for server running from
project root. Fix pagination controls showing top+bottom with per-page
selector (25/50/100/250).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/app/routers/mappings.py

@@ -1,6 +1,7 @@
 from fastapi import APIRouter, Query, Request, UploadFile, File
-from fastapi.responses import StreamingResponse, HTMLResponse
+from fastapi.responses import StreamingResponse, HTMLResponse, JSONResponse
 from fastapi.templating import Jinja2Templates
+from fastapi import HTTPException
 from pydantic import BaseModel
 from pathlib import Path
 from typing import Optional
@@ -49,15 +50,19 @@ async def mappings_page(request: Request):
 @router.get("/api/mappings")
 async def list_mappings(search: str = "", page: int = 1, per_page: int = 50,
                         sort_by: str = "sku", sort_dir: str = "asc",
-                        show_deleted: bool = False):
+                        show_deleted: bool = False, pct_filter: str = None):
     result = mapping_service.get_mappings(search=search, page=page, per_page=per_page,
                                            sort_by=sort_by, sort_dir=sort_dir,
-                                           show_deleted=show_deleted)
+                                           show_deleted=show_deleted,
+                                           pct_filter=pct_filter)
     # Merge product names from web_products (R4)
     skus = list({m["sku"] for m in result.get("mappings", [])})
     product_names = await sqlite_service.get_web_products_batch(skus)
     for m in result.get("mappings", []):
         m["product_name"] = product_names.get(m["sku"], "")
+    # Ensure counts key is always present
+    if "counts" not in result:
+        result["counts"] = {"total": 0, "complete": 0, "incomplete": 0}
     return result
 
 @router.post("/api/mappings")
@@ -67,6 +72,12 @@ async def create_mapping(data: MappingCreate):
         # Mark SKU as resolved in missing_skus tracking
         await sqlite_service.resolve_missing_sku(data.sku)
         return {"success": True, **result}
+    except HTTPException as e:
+        can_restore = e.headers.get("X-Can-Restore") == "true" if e.headers else False
+        resp: dict = {"error": e.detail}
+        if can_restore:
+            resp["can_restore"] = True
+        return JSONResponse(status_code=e.status_code, content=resp)
     except Exception as e:
         return {"success": False, "error": str(e)}