feat(app): wire bindAll() + bindAllDevSeed() dispatcher; add DI explainer page

apps/web-next/src/server/bind-production.ts now exports three functions:
- bindAllProduction() — production-only binders
- bindAllDevSeed() — dev-seed-only binders (NEW, calls all 5 features)
- bindAll() — dispatcher that branches on USE_DEV_SEED env var

All page/route callers (page.tsx, about/page.tsx, blog/[slug]/page.tsx,
api/trpc/[trpc]/route.ts) updated from bindAllProduction → bindAll so the
env flag actually has effect.

docs/architecture/di-explainer.html (NEW): standalone interactive page
explaining the di/ folder file-by-file, the loading sequence (8 stages),
the three binding kinds (.to / .toDynamicValue / .toConstantValue), an
interactive three-mode picker showing how the same blogContainer state
differs across default/dev-seed/production, a conditions table, and a
final card on how tests bypass DI entirely. Sister page to
data-flow-explainer.html.

Refactor log entry + canonical doc updates follow in subsequent commits.

apps/web-next/src/app/about/page.tsx

@@ -1,8 +1,8 @@
 import { appRouter } from "@repo/core-api";
-import { bindAllProduction } from "../../server/bind-production";
+import { bindAll } from "../../server/bind-production";
 
 export default async function AboutPage() {
-  await bindAllProduction();
+  await bindAll();
   const caller = appRouter.createCaller({});
   const page = await caller.marketingPages.pageBySlug({ slug: "about" });