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/locales/de.json

@@ -0,0 +1,19 @@
+{
+  "settings": {
+    "title": "Einstellungen",
+    "language": "Sprache",
+    "languageDescription": "Oberflächensprache von Kon. Wirkt sich nicht auf die Transkription aus."
+  },
+  "history": {
+    "title": "Verlauf",
+    "empty": "Noch keine Transkripte — drück das Kürzel und leg los."
+  },
+  "tasks": {
+    "title": "Aufgaben"
+  },
+  "dictation": {
+    "ready": "Bereit",
+    "recording": "Aufnahme…",
+    "processing": "Verarbeitung…"
+  }
+}