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/templates/shares.html

@@ -47,7 +47,7 @@
       <input type="hidden" name="orig" value="{{.Form.Orig}}">
       <div class="form-grid">
         <label>Share name <input name="name" required value="{{.Form.Name}}" placeholder="documents"></label>
-        <label>Path <input name="path" required list="mountpoints" value="{{.Form.Path}}" placeholder="/nas_pool_1_8_gb/documents"></label>
+        <label>Path <input name="path" required list="mountpoints" value="{{.Form.Path}}" placeholder="/nas_pool_1_8_gb/documents or /mnt/usb"></label>
         <label>Comment <input name="comment" value="{{.Form.Comment}}"></label>
         <label class="check"><input type="checkbox" name="read_only" value="1" {{if .Form.ReadOnly}}checked{{end}}> Read-only</label>
         <label class="check"><input type="checkbox" name="guest" value="1" {{if .Form.Guest}}checked{{end}}> Guest access (anyone on the LAN, no password)</label>
@@ -58,6 +58,7 @@
           <span class="checks">{{range .Users}}<label class="check-item"><input type="checkbox" name="users" value="{{.}}" {{if contains $.Data.Form.Users .}}checked{{end}}> {{.}}</label>{{else}}<span class="muted">no SMB users yet — add one below</span>{{end}}</span>
         </div>
       </div>
+      <p class="small muted">Path: any folder on the NAS — a ZFS dataset, a USB disk under /mnt or /media, /srv… System directories (/etc, /usr, /var…) are refused.</p>
       <p class="small muted">Files are read and written as <b>{{.Owner}}</b> regardless of the SMB user, so containers and shares see the same ownership.</p>
       <button type="submit">{{if .Form.New}}Create share{{else}}Save share{{end}}</button>
       {{if not .Form.New}}<a href="/shares" class="small">cancel</a>{{end}}
@@ -118,7 +119,7 @@
     <form method="post" action="/shares/nfs/save" class="ds-form">
       <input type="hidden" name="orig" value="{{.NForm.Orig}}">
       <div class="form-grid">
-        <label>Path <input name="path" required list="mountpoints" value="{{.NForm.Path}}" placeholder="/nas_pool_1_8_gb/documents"></label>
+        <label>Path <input name="path" required list="mountpoints" value="{{.NForm.Path}}" placeholder="/nas_pool_1_8_gb/documents or /mnt/usb"></label>
         <label>Clients <input name="clients" required value="{{.NForm.Clients}}" placeholder="192.168.101.0/24, pc.lan, *"></label>
         <label>Squash
           <select name="squash">
@@ -130,7 +131,7 @@
         <label class="check"><input type="checkbox" name="read_only" value="1" {{if .NForm.ReadOnly}}checked{{end}}> Read-only</label>
         <label class="check"><input type="checkbox" name="async" value="1" {{if .NForm.Async}}checked{{end}}> async writes (faster, less safe on power loss)</label>
       </div>
-      <p class="small muted">Nested datasets under the path are exported too (crossmnt). Clients: host names, IPs, networks like 192.168.101.0/24, or * for everyone.</p>
+      <p class="small muted">Filesystems mounted below the path (nested datasets, disks) are exported too (crossmnt). Clients: host names, IPs, networks like 192.168.101.0/24, or * for everyone.</p>
       <button type="submit">{{if .NForm.New}}Create export{{else}}Save export{{end}}</button>
       {{if not .NForm.New}}<a href="/shares" class="small">cancel</a>{{end}}
     </form>