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, 42 insertions, 0 deletions

diff --git a/web/src/keySounds.js b/web/src/keySounds.js
new file mode 100644
index 0000000..1a20087
--- /dev/null
+++ b/web/src/keySounds.js
@@ -0,0 +1,42 @@
+import { getSettings } from './settings.js';
+
+// Synthesized rather than sampled - a couple of short tones via the Web
+// Audio API instead of shipping/loading actual click-sample audio files,
+// so the feature has zero asset dependencies.
+const THEMES = {
+    click: { freq: 1800, duration: 0.03, type: 'square' },
+    mech: { freq: 180, duration: 0.05, type: 'triangle' },
+};
+
+let audioCtx = null;
+function getContext() {
+    if (!audioCtx) {
+        const AudioContextClass = window.AudioContext || window.webkitAudioContext;
+        audioCtx = new AudioContextClass();
+    }
+    return audioCtx;
+}
+
+// Browsers refuse to start an AudioContext before a user gesture - the
+// first keystroke that reaches here always follows one (focusing/typing
+// into the input), so resuming inline rather than requiring a separate
+// "enable sound" click.
+export function playKeySound() {
+    const { soundTheme } = getSettings();
+    const theme = THEMES[soundTheme];
+    if (!theme) return;
+
+    const ctx = getContext();
+    if (ctx.state === 'suspended') ctx.resume();
+
+    const osc = ctx.createOscillator();
+    const gain = ctx.createGain();
+    osc.type = theme.type;
+    osc.frequency.value = theme.freq;
+    gain.gain.setValueAtTime(0.12, ctx.currentTime);
+    gain.gain.exponentialRampToValueAtTime(0.0001, ctx.currentTime + theme.duration);
+    osc.connect(gain);
+    gain.connect(ctx.destination);
+    osc.start();
+    osc.stop(ctx.currentTime + theme.duration);
+}