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.lock

@@ -1140,7 +1140,7 @@ checksum = "384b8ab6d37215f3c5301a95a4accb5d64aa607f1fcb26a11b5303878451b4fe"
 
 [[package]]
 name = "openpxe"
-version = "0.4.65"
+version = "0.4.66"
 dependencies = [
  "anyhow",
  "axum",
@@ -1162,7 +1162,7 @@ dependencies = [
 
 [[package]]
 name = "openpxe-core"
-version = "0.4.65"
+version = "0.4.66"
 dependencies = [
  "anyhow",
  "bcrypt",
@@ -1181,7 +1181,7 @@ dependencies = [
 
 [[package]]
 name = "openpxe-dhcp-proxy"
-version = "0.4.65"
+version = "0.4.66"
 dependencies = [
  "anyhow",
  "bytes",
@@ -1195,7 +1195,7 @@ dependencies = [
 
 [[package]]
 name = "openpxe-http-api"
-version = "0.4.65"
+version = "0.4.66"
 dependencies = [
  "anyhow",
  "axum",
@@ -1226,7 +1226,7 @@ dependencies = [
 
 [[package]]
 name = "openpxe-ipxe-assets"
-version = "0.4.65"
+version = "0.4.66"
 dependencies = [
  "openpxe-core",
  "rust-embed",
@@ -1236,7 +1236,7 @@ dependencies = [
 
 [[package]]
 name = "openpxe-iso-store"
-version = "0.4.65"
+version = "0.4.66"
 dependencies = [
  "anyhow",
  "bcrypt",
@@ -1260,7 +1260,7 @@ dependencies = [
 
 [[package]]
 name = "openpxe-tftp"
-version = "0.4.65"
+version = "0.4.66"
 dependencies = [
  "anyhow",
  "bytes",
@@ -1274,7 +1274,7 @@ dependencies = [
 
 [[package]]
 name = "openpxe-webui"
-version = "0.4.65"
+version = "0.4.66"
 
 [[package]]
 name = "parking_lot"