Shares: allow any folder, not only ZFS datasets

SMB shares and NFS exports may now point to any existing directory
(USB disks under /mnt or /media, /srv, home folders...) instead of only
ZFS dataset mountpoints. System trees (/etc, /usr, /var, /boot, /proc,
/root, ...) are refused, also after resolving symlinks. The path
suggestions list mounted data filesystems from /proc/self/mounts in
addition to ZFS mountpoints.

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

internal/web/server.go

@@ -9,6 +9,7 @@ import (
 	"log"
 	"math"
 	"net/http"
+	"path/filepath"
 	"strings"
 	"sync"
 	"time"
@@ -38,7 +39,10 @@ type Server struct {
 	pages  map[string]*template.Template
 	mux    *http.ServeMux
 
-	dirExists func(string) bool // share path check; replaced in tests
+	// share path checks; replaced in tests
+	dirExists  func(string) bool
+	realPath   func(string) (string, error)
+	readMounts func() string
 
 	mu       sync.Mutex
 	shareErr string // last failed Apply of the share config
@@ -47,7 +51,8 @@ type Server struct {
 func New(cfg *config.Store, st *store.Store, ring *history.Ring, sch *sched.Scheduler, sh *shares.Store) *Server {
 	run := hostrun.Auto()
 	s := &Server{cfg: cfg, st: st, ring: ring, sch: sch, send: alert.SendMail,
-		admin: zfsadm.New(zfsadm.NoInput(run)), shares: sh, shm: shares.New(run), dirExists: statDir, mux: http.NewServeMux()}
+		admin: zfsadm.New(zfsadm.NoInput(run)), shares: sh, shm: shares.New(run),
+		dirExists: statDir, realPath: filepath.EvalSymlinks, readMounts: procMounts, mux: http.NewServeMux()}
 	s.pages = map[string]*template.Template{}
 	layout := template.Must(template.New("layout").Funcs(funcs).ParseFS(assets, "templates/layout.html"))
 	for _, name := range []string{"dashboard", "disk", "zfs", "alerts", "settings", "shares"} {