Add an English/Czech interface language switch

English stays the default; Czech is a full translation. Only the app chrome
is translated - notes and generated decks keep their own language, which is a
core feature, not a setting.

i18n
- lib/i18n-dict.ts: flat en/cs dictionaries (441 keys each), translate() and
  translatePlural() with real Czech plural rules (1 sada / 2-4 sady / 5+ sad).
  English is the default and the fallback for a missing key.
- lib/i18n.ts: browser store (freezmo.locale.v1), useI18n() hook, and the
  x-freezmo-locale header. The hook's t/plural are bound to the locale React
  is rendering with, so the first client render matches the server HTML and
  then re-renders in the stored language; module-level t is only for event
  handlers and non-React helpers.
- <html lang> follows the choice (inline script + setLocale).

UI
- Settings -> Language segmented control (English / Cestina), plus an EN/CS
  button on the key gate for first run.
- Every screen translated: sidebar, header, tab bar, library, deck cards,
  folder bar, create flow, all seven activities, study chrome, results,
  settings + health panel, API-key gate and help, install sheet, offline and
  the deck-not-found page.
- Plural-aware counts everywhere (decks, folders, activities, runs, photos,
  documents, pages, words, wrong attempts).

Server
- API routes and guards (generate, grade, health, http-guard, rate-limit,
  ollama error mapping) answer in the request's language, so error cards and
  the health panel never fall back to English mid-sentence.

Fixes
- A hydration mismatch when a non-default locale was stored silently dropped
  the .dark class after hydration; render-time t() is now bound to the
  hook's locale (UploadScreen.describeExtraction, ResultsScreen.answerFor,
  DeckCard's folder fallback).
- Bumped the service worker cache to freezmo-v4 for this deploy.

Tests: i18n 27/27 (gate toggle, persistence, html lang, Czech plurals,
settings switch) and a rebuilt regression suite 72/72 (English default,
library, create, all 7 activity types, results, settings, install sheet,
offline, PWA, security, Czech smoke).

README.md

@@ -64,7 +64,10 @@ no database, no data leaving your machine except the requests you send to Ollama
   restores it before any activities are written — the UI shows *“Restoring the notes' language…”*.
   On good transcriptions the guard stays silent and costs nothing.
 - The deck stores its language, so short answers are graded and feedback written in the same language.
-- The Freezmo interface itself stays English (only your content is language-matched).
+- The **Freezmo interface** has its own language switch: **English (default)** and a full **Czech**
+  translation, picked in **Settings → Language** or from the `EN`/`CS` button on the key gate. The
+  chosen locale is remembered per browser and also decides the language of server messages (API
+  errors, health notes), so nothing falls back to English mid-sentence.
 
 **Study feed**
 
@@ -123,7 +126,7 @@ no database, no data leaving your machine except the requests you send to Ollama
 - Deck cards: title → muted metadata → neutral tags (difficulty carries the only colour:
   green/amber/red) → summary → a single **Study** button, with everything else in the `⋯` menu
 - **Frosted UI**: the sticky header, tab bar, sheets and panels use real `backdrop-filter` blur over
-  a faint viewport-fixed aurora wash (sky/lilac/mint) — depth without noise
+  the fixed ice scene (drifting glacier blobs, glossy orbs, a whisper of grain) — depth without noise
 - **Pill language**: primary CTAs are near-black in light mode and white in dark mode; secondaries
   are outlined or ghost. Colour is reserved for meaning — activity tints (sky/lilac/mint/peach/rose),
   correct/wrong feedback, key status
@@ -839,6 +842,16 @@ Things I chose while building, with the reasoning:
     soft radial gradients with no filter — visually the same, 4-5x cheaper — phones get two blobs
     instead of four and a 16px (instead of 24px) glass blur, and nested glass never blurs. Result:
     60fps idle and ~17ms median frames while scrolling under 4x throttle.
+33. **Interface copy lives in two flat dictionaries, not in a framework.** `lib/i18n-dict.ts` holds
+    the `en` and `cs` tables plus `translate()` / `translatePlural()`; `lib/i18n.ts` adds the browser
+    store (`freezmo.locale.v1`), the `useI18n()` hook and the `x-freezmo-locale` header that API
+    routes read to answer in the same language. English is the default and the fallback for any
+    missing key. Czech uses the real plural rules (1 sada / 2–4 sady / 5+ sad). Crucially, the hook's
+    `t` is *bound to the locale React is rendering with* — the first client render matches the
+    server HTML, then re-renders in the stored language; using the module-level `t` during render
+    instead caused a hydration mismatch that silently dropped the `dark` class. Module-level `t` is
+    only for event handlers and non-React helpers (documents, folders, API errors). The notes' own
+    language is untouched by this — decks still come back in the language of your notes.
 
 14. **`gpt-oss:120b` does all content work — but it cannot read images** (`this model does not
     support image input`), so `VISION_MODEL` has to stay a multimodal model (`gemma4:31b-cloud`).