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/routes/+layout.svelte

@@ -21,9 +21,14 @@
   import { getCurrentWindow } from "@tauri-apps/api/window";
   import { listen } from "@tauri-apps/api/event";
   import { toasts } from "$lib/stores/toasts.svelte.js";
+  import { initI18n } from "$lib/i18n";
 
   import { page as sveltePage } from "$app/stores";
 
+  // Set up svelte-i18n once per app instance. Safe to call from every
+  // window — initI18n guards itself against re-init.
+  initI18n();
+
   let { children } = $props();
 
   const prefs = getPreferences();