Add update-vendor.sh symlink support, unify install.sh vendor flow

- update-vendor.sh now creates webui/vendor symlinks (htm.js)
- install.sh calls update-vendor.sh after package install
- Add vendor/.empty and webui/vendor/.empty as directory placeholders in git

lib/common.py

@@ -4,6 +4,7 @@ Provides common helpers for JSON persistence, subprocess execution,
 deep merging, and directory creation used across all subsystem modules.
 """
 
+import hashlib
 import json
 import os
 import re
@@ -12,6 +13,21 @@ from copy import deepcopy
 from pathlib import Path
 from typing import Any
 
+_APPLY_HASH_KEY = "_last_applied_hash"
+
+
+def config_hash(cfg: dict[str, Any]) -> str:
+    """Compute a SHA-256 hash of *cfg* excluding the ``_last_applied_hash`` key.
+
+    Args:
+        cfg: Config dict, possibly containing ``_last_applied_hash``.
+
+    Returns:
+        Hex digest of the stripped config JSON.
+    """
+    clean = {k: v for k, v in cfg.items() if k != _APPLY_HASH_KEY}
+    return hashlib.sha256(json.dumps(clean, sort_keys=True).encode()).hexdigest()
+
 
 def validate_interface_name(name: str) -> str:
     """Validate a Linux network interface name.
@@ -154,6 +170,8 @@ def ensure_dirs(*dirs: Path) -> None:
 
 
 __all__ = [
+    "_APPLY_HASH_KEY",
+    "config_hash",
     "deep_merge",
     "ensure_dirs",
     "load_json",