feat(files): fetch file blob by id

lib/api/files.ts

@@ -1,4 +1,33 @@
-export function getFileById(fileId: string): never {
-  void fileId
-  throw new Error("Not implemented")
+export async function getFileById(fileId: string): Promise<Blob> {
+  const res = await fetch(`/api/v1/files/${encodeURIComponent(fileId)}`)
+
+  if (res.ok) return await res.blob()
+
+  const text = await res.text()
+  let json: unknown = null
+  if (text) {
+    try {
+      json = JSON.parse(text) as unknown
+    } catch {
+      json = null
+    }
+  }
+
+  const maybeObj = (typeof json === "object" && json !== null ? json : null) as {
+    code?: unknown
+    message?: unknown
+    msg?: unknown
+  } | null
+
+  const code = (typeof maybeObj?.code === "number" ? maybeObj.code : res.status) as number
+  const msg =
+    typeof maybeObj?.message === "string"
+      ? maybeObj.message
+      : typeof maybeObj?.msg === "string"
+        ? maybeObj.msg
+        : text || res.statusText || "Request failed"
+
+  if (res.status === 401 || code === 401) throw new Error("UNAUTHORIZED")
+
+  throw { code, msg }
 }