Refuse to write a bundle whose parens we unbalanced

Commit 47cdf72 shipped a pattern that matched one closing paren and emitted one,
netting an extra `)` in the Angular bundle:

    c(2,"filterByProviders",["STORJ_IX","S3","B2"]))("required",!0)
                                                  ^^ syntax error

The TrueNAS web UI went blank. Worse, MARKER was now present in the file, so
every later run reported "already patched" and skipped -- the patch could not
heal itself, and the bundle had to be hand-restored from the .pre-truecloud-patch
backup.

patch_ui.py now compares the parenthesis balance before and after substitution and
refuses to write if it changed. A bundle we cannot patch correctly is left exactly
as it was: an unpatched UI is a missing dropdown entry, a corrupted one is a dead
web UI.

Tests cover the real regression (verbatim 47cdf72 pattern) end to end: it still
matches, the balance still shifts, main() refuses, and the file on disk is
byte-for-byte unchanged. README documents the manual recovery for anyone who
already hit it.

93 tests, ruff and shellcheck clean.

patch/patch_ui.py

@@ -65,6 +65,11 @@ def _match_pattern(content):
     return None, None
 
 
+def _paren_delta(s):
+    """Net parenthesis balance. Patching must not change it — see main()."""
+    return s.count("(") - s.count(")")
+
+
 def find_bundle():
     """
     Search WEBUI_CANDIDATES for the JS chunk containing the filterByProviders
@@ -135,6 +140,24 @@ def main():
         )
         return
 
+    # Never write JS whose parentheses we have unbalanced. A pattern that eats one
+    # paren too many is a syntax error in the bundle and the entire TrueNAS web UI
+    # goes blank -- and because MARKER is then present, every later run reports
+    # "already patched" and skips, so the patch cannot heal itself. Recovery means
+    # hand-restoring the .pre-truecloud-patch backup.
+    #
+    # This is not hypothetical: it shipped once. Refuse instead.
+    if _paren_delta(patched) != _paren_delta(content):
+        print(
+            "[truecloud-patch] ERROR: the replacement would unbalance the bundle's "
+            "parentheses — refusing to write.\n"
+            "[truecloud-patch] The UI is UNCHANGED and still works. This means the "
+            "pattern no longer fits this TrueNAS build.\n"
+            "[truecloud-patch] File an issue at "
+            "https://github.com/sudolulo/truenas-truecloud-patch"
+        )
+        return
+
     tmp = path + ".tmp"
     try:
         with open(tmp, "w", encoding="utf-8") as fh: