fix(build): wire ENABLE_MKNOD and ALLOW_MOUNT_FS into LXC features

Both settings were collected via Advanced Settings UI (Steps 26-27)
but never applied to the pct create FEATURES string. This commit:

- Add mknod=1 and mount=<fs> to FEATURES string in build_container()
- Map ALLOW_MOUNT_FS in base_settings() (was only set in advanced path)
- Add var_mknod and var_mount_fs to global VAR_WHITELIST (3rd whitelist)
- Add input validation for var_mknod (0/1) and var_mount_fs (alphanumeric+comma)
- Show mknod and mount_fs in confirmation dialog (Step 28 summary)
- Show mknod and mount_fs in terminal echo after advanced settings
- Log mknod and mount_fs to session log file

Comma-separated user input (e.g. nfs,cifs) is converted to semicolons
(mount=nfs;cifs) as required by pct features format.

CHANGELOG.md

@@ -450,12 +450,6 @@ Exercise vigilance regarding copycat or coat-tailing sites that seek to exploit
 
     - Add pnpm as a dependency to ghost-cli install [@YourFavoriteKyle](https://github.com/YourFavoriteKyle) ([#13789](https://github.com/community-scripts/ProxmoxVE/pull/13789))
 
-### 💾 Core
-
-  - #### ✨ New Features
-
-    - core: wire ENABLE_MKNOD and ALLOW_MOUNT_FS into LXC features [@MickLesk](https://github.com/MickLesk) ([#13796](https://github.com/community-scripts/ProxmoxVE/pull/13796))
-
 ## 2026-04-15
 
 ### 🆕 New Scripts