Add multiplayer bots, typing languages, and rework the UI layout

Multiplayer
- Quick match: POST /api/multiplayer/quickmatch returns whichever room is
  still filling, or opens one. Players never see a room code; joining by
  code stays for racing specific people.
- Bots fill quick-match rooms after a short wait so a new game is never an
  empty lobby. They only ever join quick-match rooms, never a room opened
  by code. One or two per room, drawn from separate ~40 and ~80 WPM tiers so
  two bots are never near each other's pace, and they stall to correct
  mistakes rather than typing a clean straight line.
- Live player count via GET /api/multiplayer/online, shown on the
  Multiplayer control and under the main menu's Multiplayer button.
- Per-racer colours: you are the theme accent, opponents take distinct hues
  that stay the same from lobby to race.
- The countdown no longer holds the room lock for its full three seconds,
  which is what reset clients mid-countdown.

Typing languages
- 16 languages for the generated-word modes, each with its own
  high-frequency vocabulary rather than a translation of the English list.
- Picker in the top-right rail; non-English uses its own list at every
  difficulty tier instead of falling back to English words.

Fix UTF-8 accuracy in the game core
- update_game_state mixed byte and character counts: total_characters_typed
  accumulated byte-length deltas while total_correct_characters compared a
  char index against that byte count. Equal on ASCII, so it went unnoticed;
  a correctly typed Spanish passage scored 6%. The old byte slicing would
  also have panicked if an index landed inside a multi-byte character.
  Rewritten char-based, with regression tests.

Programming mode
- Replaced prose about programming with real code: 26 syntax-highlighted
  snippets across JavaScript, Python, Rust, C/Go/Java and shell. Single-line
  by necessity, since the typing input is a single-line field.

Layout and readability
- One icon rail arrangement on every screen: Settings/Store under the
  wordmark, Language/Theme/Friends/Account top-right, Stats/Leaderboard/
  Multiplayer bottom-right.
- Main menu: mode picker moved out of the Single Player button, which it was
  notching a divider through and pushing the label off-centre.
- Escape returns to the menu, closing any open popover first, and confirms
  before abandoning a live race.
- Split --text-color and --sub-color per theme; they shared one value that
  measured 3.65:1 against the background, below the 4.5:1 body-text floor.
- Semantic colours used in exactly one place each: gold for a personal best,
  amber for the race countdown and the mobile-result badge.
- Passage now sits in the same place on the typing and end screens, and its
  column is a whole number of characters wide so wrapping cannot leave a
  permanent gap on the right.
- End screen: keystrokes and a correct/wrong/extra/missed split, attribution
  carried over from the typing screen, and a graph with a separate error
  axis, axis titles including seconds, and smoothed lines.

1 files changed, 75 insertions, 0 deletions

diff --git a/web/src/auth.js b/web/src/auth.js
new file mode 100644
index 0000000..d4913ca
--- /dev/null
+++ b/web/src/auth.js
@@ -0,0 +1,75 @@
+import { api, ApiError } from './api.js';
+import { mergeRemoteBests } from './pb.js';
+
+// Mirrors theme.js's pattern: one module-level value plus a subscriber set,
+// so every screen showing sign-in state (the corner rail, the main menu's
+// bottom-left widget, the Account screen) repaints from the same source
+// instead of each tracking its own copy.
+let currentUser = null;
+let resolved = false;
+const listeners = new Set();
+
+function setUser(user) {
+    currentUser = user;
+    resolved = true;
+    for (const fn of listeners) fn(currentUser);
+    // Reconciles local personal bests against the server's on every
+    // sign-in (including the session restored on page load, not just an
+    // explicit login) - a pure max(local, remote) merge, so running it
+    // every time is harmless and keeps a second device's progress visible
+    // here without a dedicated "sync now" action.
+    if (user) {
+        api.get('/api/stats/me').then(({ personal_bests }) => mergeRemoteBests(personal_bests)).catch(() => {});
+    }
+}
+
+// Fire-and-forget on module load: restores sign-in state from the session
+// cookie (if any) without the caller needing to await anything up front.
+const initial = api.get('/api/auth/me').then(user => setUser(user)).catch(() => setUser(null));
+
+export function getUser() {
+    return currentUser;
+}
+
+/** True once the initial /api/auth/me check has resolved either way. */
+export function isAuthResolved() {
+    return resolved;
+}
+
+export function onAuthChange(fn) {
+    listeners.add(fn);
+    return () => listeners.delete(fn);
+}
+
+export function whenAuthReady() {
+    return initial;
+}
+
+export async function register(username, password) {
+    const user = await api.post('/api/auth/register', { username, password });
+    setUser(user);
+    return user;
+}
+
+export async function login(username, password) {
+    const user = await api.post('/api/auth/login', { username, password });
+    setUser(user);
+    return user;
+}
+
+// Fire-and-forget: called after every finished test regardless of sign-in
+// state, so callers (endScreen.js) don't need their own "am I logged in"
+// branch. No-ops for guests; a failed upload (session expired mid-test,
+// network hiccup) is silently dropped rather than surfacing an error over a
+// result the player has already seen and moved past.
+export function submitTestResult(result) {
+    if (!currentUser) return;
+    api.post('/api/stats', result).catch(() => {});
+}
+
+export async function logout() {
+    try { await api.post('/api/auth/logout'); } catch { /* cookie may already be gone */ }
+    setUser(null);
+}
+
+export { ApiError };