SMB: map to guest = Never when no share allows guests

With "Bad User" an unknown account (the Windows login) silently became a
guest session; Windows 10/11 refuse guest sessions and report "network
path not found" instead of asking for a password, so password-only shares
were unreachable from Explorer. Map to guest only while a guest share
exists, and document the net use workaround on the shares page.

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

internal/web/templates/shares.html

@@ -80,7 +80,8 @@
     <div class="field"><span class="field-name">&nbsp;</span><span><button type="submit" class="secondary">Add user / set password</button></span></div>
   </form>
   <p class="muted small">Windows: <span class="mono">\\{{.Host}}\share</span> in Explorer, or map a network drive. macOS: Finder → Go → Connect to Server → <span class="mono">smb://{{.Host}}/share</span>.
-  Note: Windows 10/11 Pro/Enterprise refuse <i>guest</i> shares by default (error 0x80070035 «network path not found») — use an SMB user with a password, or allow guest access on the PC (gpedit: Lanman Workstation → Enable insecure guest logons).</p>
+  Note: Windows 10/11 Pro/Enterprise refuse <i>guest</i> shares by default (error 0x80070035 «network path not found») — use an SMB user with a password, or allow guest access on the PC (gpedit: Lanman Workstation → Enable insecure guest logons).
+  If a password share still gives 0x80070035, connect explicitly: <span class="mono">net use \\{{.Host}}\share /user:&lt;name&gt; *</span> (Windows keeps one login per server — run <span class="mono">net use * /delete</span> first).</p>
 </section>
 
 <section class="card" id="nfs">