fix(drop): stream every dropped file to disk; no local size cap

- POST /api/run/drop-file pipes the body to its file as it arrives in every
  case — a member's (S3) through `write-as … replace` with the bytes on
  stdin — instead of buffering up to 20 MB in memory for a remote browser.
- Remote limit (server mode, a LAN browser) 20 MB → 500 MB, as a Files upload.
- A loopback browser in local mode has no cap any more, only the data dir's
  free space less a 1 GB reserve; ETOOBIG's max_bytes is what still fits, so
  the UI's "at most N MB" stays true.
- drop-image keeps its 20 MB in-memory limit, now checked on its own.
- fake-sudo gains a spawn for run-as.__setSpawnImpl.

server/__tests__/fixtures/fake-sudo.js

@@ -1,32 +1,34 @@
 /**
  * @file fake-sudo.js
  * @description Test helper for the S3 suites: an `execImpl` for
- * `run-as.__setExecImpl` that plays `sudo -n -u <login> [VAR=v…] <bin> …`
- * without root — it records the argv (so a test can assert nothing secret
+ * `run-as.__setExecImpl` (and a `spawn` for `__setSpawnImpl`) that plays
+ * `sudo -n -u <login> [VAR=v…] <bin> …` without root — it records the argv (so a test can assert nothing secret
  * crossed sudo) and runs the real `scripts/server/write-as.sh` for
  * `/opt/agenthub/libexec/write-as`, as this account.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
 const path = require("node:path");
-const { execFileSync } = require("node:child_process");
+const { execFileSync, spawn: spawnChild } = require("node:child_process");
 
 const WRITE_AS = path.join(__dirname, "..", "..", "..", "scripts", "server", "write-as.sh");
 
-/** `{ exec, calls }`: pass `exec` to `runAs.__setExecImpl`; `calls` = every sudo argv. */
+/** `{ exec, spawn, calls }`: pass `exec` to `runAs.__setExecImpl` and `spawn`
+ *  to `runAs.__setSpawnImpl`; `calls` = every sudo argv. */
 function fakeSudo() {
   const calls = [];
-  const exec = (cmd, argv, opts = {}) => {
+  // The command sudo would run, as [file, args].
+  const unwrap = (cmd, argv) => {
     calls.push([cmd, ...argv]);
-    if (cmd !== "sudo") return execFileSync(cmd, argv, opts);
+    if (cmd !== "sudo") return [cmd, argv];
     let i = argv.indexOf("-u") + 2; // past the login
     while (/^[A-Z_][A-Z0-9_]*=/.test(argv[i])) i++;
     const [bin, ...rest] = argv.slice(i);
-    if (bin === "/opt/agenthub/libexec/write-as")
-      return execFileSync("sh", [WRITE_AS, ...rest], opts);
-    return execFileSync(bin, rest, opts);
+    return bin === "/opt/agenthub/libexec/write-as" ? ["sh", [WRITE_AS, ...rest]] : [bin, rest];
   };
-  return { exec, calls };
+  const exec = (cmd, argv, opts = {}) => execFileSync(...unwrap(cmd, argv), opts);
+  const spawn = (cmd, argv, opts = {}) => spawnChild(...unwrap(cmd, argv), opts);
+  return { exec, spawn, calls };
 }
 
 module.exports = { fakeSudo };