fix(theme): remove ember-this-fallback usage for Discourse 2026.4+ / Ember 6.10

[rapekas]

Closes #72

Problem

On Discourse 2026.4.0-latest / Ember 6.10.1, the theme bundle fails to load with:

Could not find module 'ember-this-fallback/js-component' imported from (require)

Discourse no longer ships the ember-this-fallback compatibility layer; theme templates that still go through the old this-property-fallback pipeline can emit require() calls for ember-this-fallback/js-component (and related paths like is-component).

Root cause in this theme

1. Colocated right-sidebar-blocks-below.hbs — standalone .hbs for a theme component is compiled on a path that still depends on ember-this-fallback helpers.
2. Stray {{yield}} in fkb-panel.gjs — inside the badges {{#each}}, yield is not a real block API for this component; the AST can be treated as an ambiguous path and wrapped with fallback helpers that pull in removed modules.

Changes

• Replace javascripts/discourse/components/right-sidebar-blocks-below.hbs with right-sidebar-blocks-below.gjs:
  • Use settings.right_sidebar_below_fkb_panel instead of (theme-setting ...).
  • Resolve the Right Sidebar Blocks entry with getOwner(this).resolveRegistration("component:right-sidebar-blocks") and render with <this.rightSidebarBlocks /> (no legacy .hbs; no #let needed).
• fkb-panel.gjs: remove the erroneous {{yield}} from the badge loop.

Verification

• Install theme on Discourse ≥ 2026.4 with Right Sidebar Blocks if testing right_sidebar_below_fkb_panel.
• Hard-refresh / update theme from git; confirm theme-javascripts/*.js hash changes and the orange admin banner is gone.
• Browser console: no ember-this-fallback/* errors for this theme.

References

• Upstream report: [Bug] 主题在 Discourse 2026.4.0-latest 上崩溃 — 无法找到模块 'ember-this-fallback/js-component' #72
• Discourse: Deprecating .hbs in themes and plugins
• Related: this-property-fallback / migration notes