fix(theseus/addons): package addon-tab-preload.js — was missing since 0.3.19

The preload for full-tab add-on pages (opened via api.openTab, used by
the screenshot editor) has been in the source tree since 0.3.19 but
was never declared in the electron-builder `files:` list, so packaged
Theseus builds shipped without it. Symptom: an add-on tab loads with a
DevTools error 'ENOENT, addon-tab-preload.js not found in app.asar',
window.silentmode is undefined, and any button that uses the API
(Discard's closeTab fallback, future closeTab / DOM helpers) either
misfires or falls back to a partial no-op. The image-loading path
itself does not depend on the preload, so this is orthogonal to the
'blank screenshot' bug (that one is fixed in 0.3.26 by the CDP capture
switch); a user on any build 0.3.19 – 0.3.26 needs BOTH the CDP capture
fix (already in 0.3.26 main.js) AND this preload packaged, which is
why 0.3.25 still surfaced a broken editor.

package.json

@@ -61,6 +61,7 @@
       "approval-preload.js",
       "approval.html",
       "addon-inject-preload.js",
+      "addon-tab-preload.js",
       "addons-host.js",
       "addon-updater.js",
       "addon-update-pubkeys.js",