feat(frontend): the folders are layers — an import only goes down, a module is reached through its index.ts

`local/layers` (eslint.config.js) resolves every import to its file and refuses one that goes up
(lib/ → components/ui/ → features/ → modules/ → shell/) or that reaches into another module's own
files. A module's index.ts is its public face: the stores, mounted at the root and read app-wide,
which features/ may read too; a page is still opened lazily by import(). Its contract test probes
every edge, and three mutants of the rule (off, no face check, a door for everyone) each turn it red.

Six modules got that face; the shared channel UI (about, tags, tag manager) moved to
features/channel, the filter chips and the saved-views context to features/filters, and the prefs
store's controller type moved next to the store instead of importing it from its own page. The
header's per-module controls and the eagerly loaded rails keep eight inline suppressions, each
naming why: they go when the header gets slots and every rail loads lazily.

`chunkGuard` fails the production build when a lazily imported module lands in a boot chunk (the
entry, the shell, what they import) — e.g. a page re-exported from an index.ts, which makes its
import() a silent no-op. Proved on a build with that mutant; today's build has none.

frontend/chunkGuard.ts

@@ -0,0 +1,40 @@
+import type { Plugin } from "vite";
+import { bootLeaks, type ChunkLike } from "./src/lib/chunkLeaks";
+
+// A lazily imported page, rail, dialog or player must stay out of the chunks loaded at boot (the
+// entry, the shell and what they import): one static import of it on the boot path — a module's
+// index.ts re-exporting it is the easy way — makes its import() a silent no-op. That rule used to be
+// a hope; this makes it a build failure, naming each module and the boot chunk it landed in.
+const SHELL = "src/shell/App.tsx";
+
+export function chunkGuard(): Plugin {
+  return {
+    name: "siftlode-chunk-guard",
+    apply: "build",
+    generateBundle(_options, bundle) {
+      const lazy = new Set(
+        [...this.getModuleIds()].filter(
+          (id) => (this.getModuleInfo(id)?.dynamicImporters.length ?? 0) > 0,
+        ),
+      );
+      const chunks: ChunkLike[] = Object.values(bundle).flatMap((o) =>
+        o.type === "chunk"
+          ? [
+              {
+                fileName: o.fileName,
+                isEntry: o.isEntry,
+                imports: o.imports,
+                moduleIds: o.moduleIds,
+              },
+            ]
+          : [],
+      );
+      const leaks = bootLeaks(chunks, lazy, SHELL);
+      if (leaks.length)
+        this.error(
+          `lazily imported, yet loaded at boot: ${leaks.join(", ")}. ` +
+            "Something on the boot path imports it statically (an index.ts re-export?): import it lazily there too, or not at all.",
+        );
+    },
+  };
+}