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, 38 insertions, 0 deletions

diff --git a/web/tests/helpers.py b/web/tests/helpers.py
new file mode 100644
index 0000000..8aca3f6
--- /dev/null
+++ b/web/tests/helpers.py
@@ -0,0 +1,38 @@
+"""Shared helpers for the Playwright test suite. See README.md for how to
+run these tests and what needs to already be running.
+"""
+import random
+import string
+
+FRONTEND_URL = "http://localhost:4173"
+
+
+def random_username(prefix="test"):
+    suffix = "".join(random.choices(string.ascii_lowercase + string.digits, k=8))
+    return f"{prefix}_{suffix}"
+
+
+def random_password():
+    """A fresh password per run. These accounts are throwaway, so nothing is
+    gained by a fixed literal - and a hardcoded one in a checked-in file has
+    the shape of a real credential, which trips secret scanners."""
+    return "Tp" + "".join(random.choices(string.ascii_letters + string.digits, k=16))
+
+
+def register_and_login(page, username, password=None):
+    """Registers a fresh account through the real Account screen and leaves
+    the app on the main menu, signed in."""
+    password = password or random_password()
+    page.goto(FRONTEND_URL, timeout=40000)
+    # The account entry point is the top-right identity control: "Sign In"
+    # when signed out, your username once signed in. It used to be an icon in
+    # the main menu's bottom-left cluster.
+    page.click('.auth-control button')
+    page.wait_for_timeout(300)
+    page.click('[data-action="tab-register"]')
+    page.fill('input[name="username"]', username)
+    page.fill('input[name="password"]', password)
+    page.click('.account-form button[type="submit"]')
+    page.wait_for_selector('.account-signed-in', timeout=10000)
+    page.click('[data-action="menu"]')
+    page.wait_for_timeout(300)