feat(i18n): scaffold svelte-i18n with en/es/de locales and language selector

initI18n (src/lib/i18n/index.ts) registers three locales and picks the
initial one in order: kon_locale in localStorage > navigator.language short
code > en. +layout.svelte calls it once at app boot; guarded so per-window
re-init is a no-op.

Locale files are deliberately sparse — this is a scaffolding pass so strings
can be migrated incrementally. The Settings → Appearance → Language picker
plus its own description is the first real consumer; everything else
continues to render as hardcoded text until extracted.

Also: split the @chenglou/pretext ambient shim into src/lib/shims.d.ts. The
declaration previously lived in app.d.ts alongside a top-level `export {}`,
which made app.d.ts a module — scoping `declare module` to its own imports
and breaking resolution from src/lib/utils/textMeasure.ts. The fresh
.svelte-kit sync triggered by installing svelte-i18n surfaced it. Ambient
shim files must stay script-scoped (no top-level imports/exports).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/i18n/index.ts

@@ -0,0 +1,73 @@
+//! i18n scaffolding via svelte-i18n.
+//!
+//! Scope for the initial pass: wire the stack (loader, persisted choice,
+//! Settings selector) so strings can be migrated incrementally. Only a
+//! handful of labels are actually translated today — everything else
+//! continues to render as-is via hardcoded text until extracted.
+//!
+//! Locales currently shipped: en (source of truth), es, de. Adding a new
+//! locale is one `register` call + a new `locales/<code>.json`.
+
+import { init, register, locale as svelteLocale } from "svelte-i18n";
+import { derived, get } from "svelte/store";
+
+export type Locale = "en" | "es" | "de";
+
+export const SUPPORTED_LOCALES: { code: Locale; label: string }[] = [
+  { code: "en", label: "English" },
+  { code: "es", label: "Español" },
+  { code: "de", label: "Deutsch" },
+];
+
+const STORAGE_KEY = "kon_locale";
+
+register("en", () => import("./locales/en.json"));
+register("es", () => import("./locales/es.json"));
+register("de", () => import("./locales/de.json"));
+
+function detectInitialLocale(): Locale {
+  if (typeof localStorage !== "undefined") {
+    const stored = localStorage.getItem(STORAGE_KEY);
+    if (stored && SUPPORTED_LOCALES.some((option) => option.code === stored)) {
+      return stored as Locale;
+    }
+  }
+  if (typeof navigator !== "undefined" && navigator.language) {
+    const short = navigator.language.split("-")[0] as Locale;
+    if (SUPPORTED_LOCALES.some((option) => option.code === short)) {
+      return short;
+    }
+  }
+  return "en";
+}
+
+let initialised = false;
+
+export function initI18n(): void {
+  if (initialised) return;
+  initialised = true;
+
+  init({
+    fallbackLocale: "en",
+    initialLocale: detectInitialLocale(),
+  });
+}
+
+export function setLocale(code: Locale): void {
+  svelteLocale.set(code);
+  if (typeof localStorage !== "undefined") {
+    try {
+      localStorage.setItem(STORAGE_KEY, code);
+    } catch {
+      // non-fatal: private browsing, quota, etc.
+    }
+  }
+}
+
+export const currentLocale = derived(svelteLocale, ($locale) =>
+  ((($locale ?? "en").split("-")[0]) as Locale),
+);
+
+export function getCurrentLocale(): Locale {
+  return get(currentLocale);
+}