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/migrations/20260825075218_add_asset_item_area/migration.sql

@@ -0,0 +1,4 @@
+-- Hand-written (same deviation as other 2026-08-2x migrations -- see CLAUDE.md).
+ALTER TABLE "asset_items" ADD COLUMN "area_id" UUID;
+ALTER TABLE "asset_items" ADD CONSTRAINT "asset_items_area_id_fkey"
+    FOREIGN KEY ("area_id") REFERENCES "areas"("id") ON DELETE SET NULL ON UPDATE CASCADE;