Add AssetItem.areaId, show assigned-to + area on /assets, mm/dd/yyyy dates

AssetItem.areaId reuses the existing Area entity (built for InventoryScan/
the office-move feature) rather than a free-text location string -- same
share-the-lookup-entity precedent floated for Department. Real-world
coverage from the ITAM export is thin: Physical location was the literal
string DAN for all 613 rows (useless), only Stockroom carried signal, and
only for the ~71 in_stock items -- an assigned item is with a person, not
in a stockroom bin, so most rows have no area by design, not by gap.

/assets page: the assigned employee (via the active, non-returned
Assignment) was already in the DB from the 2026-08-24 backfill but never
shown -- added an Assigned To column, folded into the free-text search
too. Added an Area column + filter dropdown. Removed the Created At
column (not actionable) and format Purchase Date/Warranty End Date as
mm/dd/yyyy instead of raw ISO strings.

CSV import/export: area column added (resolved/created via
createOrGetArea(), same idempotent-by-name helper the bots Area picker
uses -- not a raw UUID). Re-ran the 613-row import after this: 0 created,
613 updated, 0 errors -- backfilled area onto the 71 rows with a
Stockroom value (46 IT Cabinet, 21 Stockroom, 4 For allocation), verified
against the source file.

Hand-written migration, no Vitest coverage -- same deviation as the rest
of this branch, see CLAUDE.md. tsc/lint clean.

prisma/schema.prisma

@@ -151,6 +151,17 @@ model AssetItem {
   // amounts in the millions with room to spare.
   cost               Decimal?             @db.Decimal(14, 2)
   costCurrency       String?              @map("cost_currency")
+  // Reuses the Area entity built for InventoryScan/the office-move feature
+  // (2026-08-25) rather than a free-text location string -- same "share
+  // the lookup entity across tables" precedent floated for Department.
+  // Nullable: real-world coverage is thin (the ITAM export's own location
+  // data was nearly useless -- "Physical location" was the literal string
+  // "DAN" for all 613 rows, no per-item detail; only "Stockroom" carried
+  // any signal, and only for the ~71 in_stock items -- an assigned item is
+  // with a person, not sitting in a stockroom bin, so it has no area here
+  // by design, not by data gap).
+  areaId             String?              @map("area_id") @db.Uuid
+  area               Area?                @relation(fields: [areaId], references: [id], onDelete: SetNull)
   purchaseOrderId    String?              @map("purchase_order_id") @db.Uuid
   purchaseOrder      PurchaseOrder?       @relation(fields: [purchaseOrderId], references: [id])
   // Free-text PO number from an external system (e.g. the real ITAM
@@ -548,6 +559,8 @@ model Area {
   team           String?
   createdAt      DateTime        @default(now()) @map("created_at") @db.Timestamptz(6)
   inventoryScans InventoryScan[]
+  // Back-relation for AssetItem.areaId -- see that field's comment.
+  assetItems     AssetItem[]
 
   @@map("areas")
 }