feat: add CSV import/export for gear collection

Adds export (GET /api/items/export) and import (POST /api/items/import) routes
backed by a pure csv.service with no external deps, plus useExportItems/useImportItems
hooks and an Import/Export section in the Settings page.

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

tests/routes/items.test.ts

@@ -144,4 +144,64 @@ describe("Item Routes", () => {
 		});
 		expect(res.status).toBe(404);
 	});
+
+	it("GET /api/items/export returns CSV with correct content-type", async () => {
+		// Create an item first
+		await app.request("/api/items", {
+			method: "POST",
+			headers: { "Content-Type": "application/json" },
+			body: JSON.stringify({
+				name: "Tent",
+				weightGrams: 1200,
+				priceCents: 35000,
+				categoryId: 1,
+			}),
+		});
+
+		const res = await app.request("/api/items/export");
+		expect(res.status).toBe(200);
+		expect(res.headers.get("Content-Type")).toContain("text/csv");
+
+		const text = await res.text();
+		const lines = text.split("\n");
+		expect(lines[0]).toBe(
+			"name,quantity,weightGrams,priceCents,category,notes,productUrl",
+		);
+		expect(lines.length).toBeGreaterThanOrEqual(2);
+		expect(lines[1]).toContain("Tent");
+	});
+
+	it("POST /api/items/import with CSV file creates items", async () => {
+		const csvContent = [
+			"name,quantity,weightGrams,priceCents,category,notes,productUrl",
+			"Sleeping Bag,1,800,25000,Camping,,",
+		].join("\n");
+
+		const formData = new FormData();
+		formData.append(
+			"file",
+			new Blob([csvContent], { type: "text/csv" }),
+			"import.csv",
+		);
+
+		const res = await app.request("/api/items/import", {
+			method: "POST",
+			body: formData,
+		});
+
+		expect(res.status).toBe(200);
+		const body = await res.json();
+		expect(body.imported).toBe(1);
+		expect(body.errors).toHaveLength(0);
+	});
+
+	it("POST /api/items/import with no file returns 400", async () => {
+		const res = await app.request("/api/items/import", {
+			method: "POST",
+			headers: { "Content-Type": "application/json" },
+			body: JSON.stringify({}),
+		});
+
+		expect(res.status).toBe(400);
+	});
 });