fix(web): CSS.escape() the owned key before it becomes a selector

container.dataset.ownedKeys round-trips schema property keys through a
DOM dataset attribute, and the takeover handoff spliced each one
straight into '[data-child-key="' + k + '"]' with no escaping --
inconsistent with this codebase's own convention elsewhere
(plugin-file-manager.js, app-shell.js's escapeCssSelector) for building
a selector from a dynamic value. A key containing a quote or backslash
would break the selector or be steerable; Codacy's static analysis
flagged this pattern (1 high ErrorProne finding on PR #569, current
head at the time) as a new issue, though its dashboard is unreachable
from this sandbox (egress to app.codacy.com is blocked) and the
check-run API returned no detail text -- verified and fixed by reading
the diff directly rather than the tool's own description.

Added a source-assertion regression test alongside this file's
existing ones (this behavior lives in an inline script no Python test
executes).

Full suite: 4888 passed, 62 skipped, 2 failed -- both the pre-existing
Europe/Kiev/Asia/Calcutta tzdata-alias gap in this sandbox, identical
on origin/main, unrelated to this change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web_interface/templates/v3/partials/plugin_config.html

@@ -147,8 +147,13 @@
                         var owned = (container.dataset.ownedKeys || '')
                             .split(',').filter(function (k) { return k; });
                         owned.forEach(function (k) {
+                            // CSS.escape: k is a schema property key read
+                            // back off dataset.ownedKeys, not a literal --
+                            // building the attribute selector by raw
+                            // concatenation would break (or be steerable)
+                            // on a key containing a quote or backslash.
                             var node = fallback.querySelector(
-                                '[data-child-key="' + k + '"]');
+                                '[data-child-key="' + CSS.escape(k) + '"]');
                             if (node) { node.remove(); }
                         });
                         // Nothing left but the section heading, or a widget