fix: contain file-explorer writes and browse reachable out-of-base binds (#1465)

The file-explorer editor save resolved a dangling symlink leaf to the link
path and wrote through it with a plain writeFile, which followed the link and
created a file outside the bind/stack root. Reject a resolved leaf that is
itself a symlink (mirroring the managed-stack guard) and promote the save
through the atomic stage-and-rename helper, so the editor save matches its
documented atomicity and can never leave a partial file or land outside the
root.

Bind-root discovery reported every source outside the compose base as
unreachable without probing it, so a config directory mounted into both the
app and the Sencho container was wrongly non-browsable. Probe the declared
source as Sencho actually sees it; dangerous host roots, docker-socket mounts,
and managed-area overlaps stay blocked, and the dangerous classification also
reads the literal declared source so it holds across platforms.

backend/src/services/FileSystemService.ts

@@ -1376,6 +1376,22 @@ export class FileSystemService {
       throw Object.assign(new Error('Symlink escapes root directory'), { code: 'SYMLINK_ESCAPE' });
     }
 
+    // A dangling symlink leaf realpath()s to ENOENT (its target is missing), so
+    // the reattach branch above rebuilds the path with the link name intact and
+    // it passes the lexical containment check. lstat does not follow the final
+    // component, so it tells a not-yet-created file (ENOENT, allowed) apart from
+    // a dangling link (a symlink, rejected) that a follow-on writeFile/readFile
+    // would traverse out of the root. Mirrors assertRealWithinBase's guard.
+    let leafIsSymlink = false;
+    try {
+      leafIsSymlink = (await fsPromises.lstat(realTarget)).isSymbolicLink();
+    } catch (err: unknown) {
+      if ((err as NodeJS.ErrnoException).code !== 'ENOENT') throw err;
+    }
+    if (leafIsSymlink) {
+      throw Object.assign(new Error('Symlink escapes root directory'), { code: 'SYMLINK_ESCAPE' });
+    }
+
     return realTarget;
   }
 
@@ -1696,7 +1712,11 @@ export class FileSystemService {
       }
     }
 
-    await fsPromises.writeFile(safePath, content, 'utf-8');
+    // Promote through the atomic stage-and-rename helper so a crash or write
+    // failure leaves the previous target intact rather than a truncated file.
+    // rename also replaces a (rejected upstream) symlink leaf in place instead
+    // of following it, so the write can never land outside the root.
+    await this.writeStackFileAtomic(safePath, content);
     const newStat = await fsPromises.stat(safePath);
     return { ok: true, mtimeMs: newStat.mtimeMs };
   }