Fix data loss: copying a file onto itself via two sessions truncated it to 0

Two panels on the same server hold two SSH sessions by design, and every
self-copy guard compared sessionId — so same-server same-path copies slipped
through all of them, and createWriteStream truncated the destination before
the read began: the file (or every file in a folder) became 0 bytes, with the
overwrite dialog making it look like a legitimate replace.

Fix, defense in depth, keyed on FILESYSTEM identity instead of session:
- fsKey on every provider: 'local', or resolvedIP:port for SFTP (host is
  DNS-resolved at connect so hostname-vs-IP aliases compare equal; username
  excluded — same host:port is the same filesystem regardless of login)
- engine guard blocks equal/nested paths when fsKeys match; per-file
  belt-and-suspenders check right before streams open
- probe layer for hidden aliases (different addresses, same machine): when
  fsKeys differ but paths collide, a token file written via dst and read via
  src decides; visible => same machine => blocked. SFTP exposes no dev/inode
  (what cp/rsync use), so probing is the only deterministic test; fails open
  and cleans up after itself
- UI: same-folder toast now fires across two sessions to one server, and the
  overwrite dialog auto-skips (never offers Replace) when the colliding item
  IS the source file

Known residual, documented in README 'Same-file protection': one filesystem
mounted at different paths on two connections is undetectable client-side.

Regression suite against the real box: file-onto-itself and folder-onto-itself
blocked with contents byte-intact, folder-into-descendant blocked, legit
same-server different-dir copies still work, local same-path still blocked,
simulated alias caught by the probe with cleanup verified.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/bun/fs/local.ts

@@ -24,6 +24,7 @@ function toEntry(name: string, full: string, st: Stats): FileEntry {
 
 export class LocalProvider implements FileSystemProvider {
 	readonly kind = "local" as const;
+	readonly fsKey = "local";
 
 	join = (...parts: string[]) => path.join(...parts);
 	dirname = (p: string) => path.dirname(p);