fix(js): resolve ESLint no-undef warnings across 6 JS files

Three distinct patterns:

1. Vendor library globals — htmx is injected by <script> before these
   extension files load; ESLint lints files in isolation and doesn't know.
   Fix: add /* global htmx */ to htmx-sse.js and htmx-json-enc.js.

2. Cross-file globals — showNotification is defined as window.showNotification
   in app.js/notification.js but called bare in app.js and error_handler.js.
   ESLint doesn't connect window.X = Y with a bare call to X.
   Fix: add /* global showNotification */ to app.js and error_handler.js.

3. Forward-reference window.* functions — in array-table.js, checkbox-group.js,
   and custom-feeds.js, functions like removeArrayTableRow are called early
   inside event-handler closures but assigned to window.* later in the file.
   At runtime this works (the handler fires after the assignment), but ESLint
   sees the bare name at the call site.
   Fix: change bare calls to window.removeArrayTableRow(this) etc. so the
   reference is explicit and ESLint-safe.

Also guard the updateSystemStats call in app.js reconnectSSE: the function
is called but defined nowhere in the codebase. Guard with typeof check so
it won't throw ReferenceError if the reconnect path is hit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web_interface/static/v3/js/htmx-json-enc.js

@@ -1,3 +1,4 @@
+/* global htmx */
 htmx.defineExtension('json-enc', {
     onEvent: function (name, evt) {
         if (name === "htmx:configRequest") {