Improve lighthouse score

[kamilmielnik]

Closes #412

Loading & first paint

• The app is now fully server-rendered, so the board is visible immediately instead of after all JavaScript loads.
• All CSS is inlined into the HTML, removing render-blocking stylesheet requests.
• Layout sizing moved from JavaScript to CSS variables, so the first paint needs no layout code at all.
• The board background SVG is built with plain strings instead of react-dom/server, shrinking the framework chunk from 116KB to 60KB.
• The 192KB transliteration package was replaced with built-in Unicode normalization.
• Translations ship as one chunk per language, with only English in the main bundle.
• Flag icons moved out of the main bundle.
• Modals are code-split and their chunks download in the background once the page is idle.
• A modern-browser target list plus removal of Next.js's built-in polyfills dropped all legacy JavaScript from the bundles.
• Components barrel file was removed so the bundler can tree-shake properly.

Runtime performance

• Board cells no longer subscribe to the Redux store (about 2,000 subscriptions removed), which cut the main hydration task roughly in half.
• Booting with nothing saved no longer re-renders the board after hydration.
• Modals only mount the first time they are opened.
• Tooltips skip their positioning machinery until after page load, and skip it entirely on touch devices.
• The service worker registers after load and idle, so its precache downloads never compete with the page.
• The dictionary download waits for idle or the first user interaction, and starts immediately when a new language is picked.
• Assorted mount-time waste was removed: no forced reflows, cached media query objects, and no redundant localStorage write-back after boot.

Features & UX

• Returning users with a saved non-default game or language no longer see a flash: a tiny pre-paint script sizes the board and sets text direction before first paint, and the content stays hidden until the saved setup is applied.
• The active language's translations are cached in localStorage, so returning users see their language in the very first paint.
• All languages preload in the background, so switching language applies instantly.
• The "Hidden" coordinates option was removed because it caused a layout jump on load; saved settings migrate to "Original".
• Dictionary lookups skip the debounce delay when the definition is already cached.
• Added robots.txt and llms.txt.

Robustness

• Corrupt saved data (board, rack, or settings) is detected and discarded instead of breaking the app.
• A failure while restoring saved state can no longer kill solving or disable saving for the session.
• A failed translation download quietly falls back to English instead of erroring.

Lighthouse

• Before vs now