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/pages/SettingsPage.svelte

@@ -15,6 +15,8 @@
   import { clampTextLines } from "$lib/utils/textMeasure.js";
   import { bodyPretextLineHeight, pretextFontShorthand } from "$lib/utils/accessibilityTypography.js";
   import { Check, ChevronRight } from "lucide-svelte";
+  import { _ } from "svelte-i18n";
+  import { SUPPORTED_LOCALES, setLocale, currentLocale } from "$lib/i18n";
 
   const prefs = getPreferences();
 
@@ -1603,6 +1605,25 @@
                 data-no-transition
               />
             </div>
+
+            <div class="mb-2">
+              <p class="text-[10px] font-medium text-text-tertiary uppercase tracking-wider mb-2">
+                {$_("settings.language")}
+              </p>
+              <div class="inline-flex bg-bg-elevated rounded-[10px] p-[3px] gap-[2px]">
+                {#each SUPPORTED_LOCALES as option}
+                  <button
+                    class="rounded-lg font-medium px-3.5 py-[6px] text-[12px]
+                      {$currentLocale === option.code
+                        ? 'bg-accent text-bg shadow-[0_1px_4px_rgba(232,168,124,0.3)]'
+                        : 'text-text-secondary hover:text-text hover:bg-hover'}"
+                    style="transition-duration: var(--duration-ui)"
+                    onclick={() => setLocale(option.code)}
+                  >{option.label}</button>
+                {/each}
+              </div>
+              <p class="text-[11px] text-text-tertiary mt-2">{$_("settings.languageDescription")}</p>
+            </div>
           </div>
         {/if}
       </div>