[2.x] fix(flags): drop literal 'undefined' from Inappropriate reason and lazy-load FlagPostModal

[imorland]

Summary

Two related changes to the Flarum-Flags "Inappropriate" flag-post path:

1. Fix {undefined} rendering when no Community Guidelines URL is set. With no URL configured, the modal currently renders:

   This post is offensive, abusive, or violates our {undefined}.

   The reason: FlagPostModal passes undefined as the <a> rich-placeholder value to the translator, which stringifies it (the translator wraps undefined args as '{undefined}' at Translator.tsx:131, and <a> is not in the auto-provided element fallback list).

   Use a separate translation key (reason_inappropriate_text_no_guidelines) when the URL is unset, instead of trying to pass undefined as a placeholder. Matches the reporter's suggested wording ("This post is offensive or abusive.").

2. Lazy-load FlagPostModal. The modal was imported eagerly via a side-effect import './components/FlagPostModal' in forum.ts plus a static import FlagPostModal from ... in addFlagControl.js, so its ~4 KB was always shipped in the main forum bundle even for users who never click flag. Switch to the documented lazy-load pattern:

   <Button onclick={() => app.modal.show(() => import('./components/FlagPostModal'), { post })}>

   forum.js drops from 12,454 → 11,509 bytes; FlagPostModal.js becomes a deferred ~4 KB chunk loaded on first click.

   Required follow-up: declare ->jsDirectory(__DIR__.'/js/dist/forum') on the Extend\Frontend('forum') registration so Flarum's asset publisher copies the chunked sub-bundles into assets/extensions/flarum-flags/forum/components/. Without this, the chunk requests 404.

Backwards compatibility (lazy-load)

FlagPostModal is no longer in the registry at boot time — it's only registered when the lazy chunk loads (first flag-button click). Third-party extensions using the string-path form of extend() (extend('flarum/flags/components/FlagPostModal', 'view', fn)) are unaffected: the registry's onLoad hook applies pending patches when the chunk lands (common/extend.ts:36-43).

Extensions using the direct-prototype form (import FlagPostModal from 'flarum/flags/...'; extend(FlagPostModal.prototype, ...)) at initializer-time would break, because the import would resolve to undefined until the chunk loads. The string-path form is the documented recommended pattern for cross-extension extension specifically because of this; any extension targeting 2.x's registry-based module model should already be using it.

Test plan

• Unset Community Guidelines URL → flag modal Inappropriate option reads "This post is offensive or abusive." (no {undefined}).
• Set Community Guidelines URL → reads "This post is offensive, abusive, or violates our [community guidelines]." with working link.
• Initial page load: no FlagPostModal*.js request in Network tab.
• First flag-button click: chunk requests successfully from assets/extensions/flarum-flags/forum/components/FlagPostModal.js; modal renders.
• Second click: no additional chunk fetch (already loaded).
• Submit flag → flow completes normally.

Closes #4574