v0.4.66: ship smbclient in the runtime image

v0.4.65 added the SmbShareManager but the Dockerfile only installed
the `samba` package — in Debian 12 that ships the SERVER (smbd) only,
not the `smbclient` CLI the new manager shells out to. Every "Add
share" attempt surfaced:

    could not exec smbclient: No such file or directory (os error 2)

Fix is two lines: add `smbclient` to the runtime apt install, drop
the leftover `nfs-common` (no kernel-mount NFS anymore so the helpers
aren't needed).

While in the area, harden the manager so future stripped-down runtime
images get a useful error instead of a bare exec failure:

- `list_isos` and `stream_iso` both detect `ErrorKind::NotFound` on
  spawn and emit "smbclient binary not found on $PATH".
- `hint_for` translates the missing-binary pattern into an actionable
  hint: "pull OpenPXE v0.4.66+ or add the Debian `smbclient` package
  to your runtime stage." So even on a custom build the UI still
  surfaces a clear remediation.

Tests: 150 passing (+1 for the new hint). clippy clean.

The image is still ~98 MB — `smbclient` adds <1 MB on top of the
already-installed samba server.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Cargo.toml

@@ -12,7 +12,7 @@ members = [
 ]
 
 [workspace.package]
-version = "0.4.65"
+version = "0.4.66"
 edition = "2021"
 rust-version = "1.95"
 license = "MIT OR Apache-2.0"