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/shims.d.ts

@@ -0,0 +1,39 @@
+// Ambient module shims. Must stay script-scoped (no top-level imports /
+// exports) so `declare module` registers globally; adding an import/export
+// turns the file into a module and scopes the declaration away.
+
+declare module "@chenglou/pretext" {
+  export interface PretextLayoutLine {
+    text: string;
+  }
+
+  export interface PretextLayoutResult {
+    height: number;
+    lineCount: number;
+    lines: PretextLayoutLine[];
+  }
+
+  export function prepare(
+    text: string,
+    font: string,
+    options?: Record<string, unknown>,
+  ): unknown;
+
+  export function prepareWithSegments(
+    text: string,
+    font: string,
+    options?: Record<string, unknown>,
+  ): unknown;
+
+  export function layout(
+    prepared: unknown,
+    maxWidth: number,
+    lineHeight: number,
+  ): PretextLayoutResult;
+
+  export function layoutWithLines(
+    prepared: unknown,
+    maxWidth: number,
+    lineHeight: number,
+  ): PretextLayoutResult;
+}