Fix password prompt silently stripping leading/trailing whitespace

Reported live: mount error(79) again despite already switching to real
credentials + sec=ntlmssp — this time with a Samba password containing
special characters. Root cause confirmed directly: `read -r -s pw1`
without `IFS=` silently strips leading/trailing whitespace even when
reading into a single variable (verified: " P@ss word! " -> "P@ss word!",
10 chars instead of 12). A password with a leading/trailing space —
common from a password manager's copy-paste, or a stray keystroke — got
quietly trimmed on the way into the credentials file, so it no longer
matched what was actually set on the Samba account. That mismatch
surfaces as this same cryptic ENOKEY mount error, not an obvious "wrong
password".

Fixed with IFS= on both reads. Also echo the captured length (never the
password itself) right after entry, so a silently-stripped character is
something you can catch and cross-check yourself before the mount even
attempts, instead of only after it fails.

services/vpn-data-mount.sh

@@ -293,11 +293,23 @@ _vdm_find_existing_smb_password() {
 _vdm_prompt_password() {
     local prompt="$1" pw1="" pw2=""
     while true; do
+        # IFS= matters here, not just -s/-r: plain `read -r pw1` (no IFS=)
+        # silently strips leading/trailing whitespace even into a single
+        # variable — confirmed live, a password with a leading/trailing
+        # space (copy-pasted from a password manager, a stray keystroke)
+        # got quietly trimmed on the way in, so the credentials file ended
+        # up holding a DIFFERENT password than the one actually set on the
+        # Samba account. That surfaces as a cryptic mount failure, not an
+        # obvious "wrong password" — nothing here could tell the two apart.
         echo -n "  ${prompt}: " >&2
-        read -r -s pw1; echo "" >&2
+        IFS= read -r -s pw1; echo "" >&2
         echo -n "  Confirm: " >&2
-        read -r -s pw2; echo "" >&2
+        IFS= read -r -s pw2; echo "" >&2
         if [ -n "$pw1" ] && [ "$pw1" = "$pw2" ]; then
+            # Length only, never the password itself — lets you catch a
+            # silently-stripped character (or a typo) yourself before the
+            # mount attempt fails with an unhelpful error.
+            echo "  Captured (${#pw1} characters)." >&2
             printf '%s' "$pw1"
             return 0
         fi