feat: add invalid_users directive for Samba shares

Samba shares now support an 'invalid users' list (deny list), written
as 'invalid users = u1,u2' in smb.conf. The UI shows a ChipPicker
for valid_users and invalid_users, mutually exclusive, sourced from
the system user list.

feat: add ImportSystemUsers for fresh installations

When NASCTL_IMPORT_ON_BOOT=true, nasctl now imports existing system
users from /etc/passwd (UID 1000-60000) and /etc/group (supplemental
groups), and detects which have Samba accounts via 'pdbedit -L'.
Imported users are marked dirty so the admin can review before applying.
New POST /api/import/users endpoint for manual re-import.

This mirrors the existing import-on-boot flow for smb.conf and /etc/exports.

Makefile

@@ -1,5 +1,5 @@
 BINARY=nasctl
-VERSION?=0.5.3
+VERSION?=0.6.0
 GO?=go
 LDFLAGS=-s -w -X github.com/darroyo/nasctl/internal/web.Version=$(VERSION) -X github.com/darroyo/nasctl/internal/web.Commit=$(shell git rev-parse --short HEAD 2>/dev/null || echo unknown)
 BUILD_FLAGS=CGO_ENABLED=0