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.

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diff --git a/web/tests/README.md b/web/tests/README.md
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+# TyperPunk web test suite
+
+Playwright end-to-end tests covering the major user-facing flows: typing a
+test through to the end screen, account/store/cosmetics, and
+leaderboard/friends. These replace the ad-hoc scripts written into `/tmp`
+during development - checked in here so they survive past one session and
+can be re-run for regressions.
+
+## Prerequisites
+
+- Python 3 with the `playwright` package installed (`pip install
+  playwright && playwright install chromium`, or on Arch,
+  `pacman -S python-playwright`).
+- The web app's dev server running on port 4173:
+  `cd web && node serve.mjs`
+- The backend running on port 8787, pointed at a throwaway database (do not
+  run tests against a production database):
+  `DATABASE_URL="sqlite:///tmp/typerpunk_test.db" PORT=8787 COOKIE_SECURE=0 \
+    cargo run --package typerpunk-server`
+
+Both servers must already be running before starting the suite - it does
+not launch or manage them itself.
+
+## Running
+
+```
+cd web/tests
+python3 run_all.py
+```
+
+Each `test_*.py` file exposes a single `run()` function that raises
+`AssertionError` on failure. `run_all.py` imports and calls each one in
+turn, continuing after a failure so one broken test doesn't hide the
+others, and exits non-zero if anything failed.
+
+To run a single file directly: `python3 test_typing_flow.py`.
+
+## Adding a test
+
+Add a new `test_*.py` file with a `run()` function, using Playwright's sync
+API (see the existing files for the pattern). `run_all.py` discovers files
+by the `test_*.py` name, so no registration step is needed.