fix(spawn): attribute ENOMEM and ENOENT-under-memory-pressure spawn failures to host OOM (#1111)

Operators previously saw "spawn docker ENOENT" or "spawn /bin/sh ENOENT" when
the host was under memory pressure, which sent them down a missing-binary
debugging path. Linux libuv's posix_spawn can fail to allocate its argv /
path-search arena under low free memory and surface the underlying ENOMEM
as ENOENT.

Centralizes spawn-error mapping in a new utils/spawnErrors.ts helper:
- Explicit ENOMEM is rewritten to "Out of memory while launching <command>
  (host free memory: X MiB of Y MiB)".
- ENOENT under the 128 MiB free-memory floor is rewritten with the same
  wording plus a "reported as ENOENT under memory pressure" hint.
- ENOENT for docker on a healthy host preserves the existing
  "Docker CLI unavailable on this node" mapping.
- Other errors pass through unchanged.

Applied at the four named offenders: ComposeService.execute(),
ComposeService.captureCompose(), DockerController.getContainersByStack(),
and FileSystemService.getStacks() (which gets an ENOMEM-aware log line
for the scandir failure).

Startup also logs host free/total MiB once and warns when free memory is
below the 128 MiB floor, so the diagnostic surfaces before the first
spawn attempt rather than after it fails.

37 tests cover the mapping function directly and the ComposeService /
FileSystemService integration paths.

backend/src/services/FileSystemService.ts

@@ -1,4 +1,5 @@
 import path from 'path';
+import os from 'os';
 import { promises as fsPromises, createReadStream } from 'fs';
 import type { Readable } from 'stream';
 import { NodeRegistry } from './NodeRegistry';
@@ -135,7 +136,12 @@ export class FileSystemService {
 
       return stackNames;
     } catch (error: any) {
-      console.warn(`[FileSystemService] Failed to list stacks: ${error.message}`);
+      if (error?.code === 'ENOMEM') {
+        const freeMiB = Math.round(os.freemem() / (1024 * 1024));
+        console.warn(`[FileSystemService] Failed to list stacks: ENOMEM (host free memory: ${freeMiB} MiB). Returning empty list.`);
+      } else {
+        console.warn(`[FileSystemService] Failed to list stacks: ${error.message}`);
+      }
       return [];
     }
   }