Remove the RepoISO integration; keep generic support for shares of other services

RepoISO now manages the NAS itself, so nasmon no longer polls it (settings field, dashboard card,
menu link and repoiso:* alerts are gone). Drop-in SMB includes, read-only listing and keeping
smbd/nfs-server running for other software's shares stay.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/shares/external.go

@@ -7,7 +7,7 @@ import (
 	"strings"
 )
 
-// Other services on the host (e.g. RepoISO) publish their own shares through
+// Other services on the host publish their own shares through
 // drop-in files. nasmon includes the SMB drop-ins into the smb.conf it
 // generates and keeps smbd / nfs-server running while any of them define a
 // share, even when nasmon itself has none.
@@ -19,7 +19,7 @@ const (
 // ExternalShare is a share defined outside nasmon (read-only in the UI).
 type ExternalShare struct {
 	Kind    string // smb | nfs
-	Source  string // drop-in file name without extension, e.g. "repoiso"
+	Source  string // drop-in file name without extension
 	Name    string // SMB share name (empty for NFS)
 	Path    string
 	Clients string // NFS clients with options, as written in the file