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, 17 insertions, 0 deletions

diff --git a/data/packs/nature.json b/data/packs/nature.json
new file mode 100644
index 0000000..4bb274f
--- /dev/null
+++ b/data/packs/nature.json
@@ -0,0 +1,17 @@
+[
+  {"category":"nature","content":"The world is big and I want to have a good look at it before it gets dark.","attribution":"John Muir"},
+  {"category":"nature","content":"In every walk with nature one receives far more than he seeks.","attribution":"John Muir"},
+  {"category":"nature","content":"Heaven is under our feet as well as over our heads.","attribution":"Henry David Thoreau, Walden"},
+  {"category":"nature","content":"The Amazon rainforest is home to an estimated ten percent of all known species on Earth, making it one of the most biodiverse regions on the planet.","attribution":"Nature"},
+  {"category":"nature","content":"Coral reefs cover less than one percent of the ocean floor, yet they support roughly a quarter of all marine species.","attribution":"Nature"},
+  {"category":"nature","content":"Migratory birds can travel thousands of miles between breeding and wintering grounds, navigating using the sun, stars, and the Earth's magnetic field.","attribution":"Nature"},
+  {"category":"nature","content":"A single mature tree can absorb around twenty two kilograms of carbon dioxide per year, making forests an important part of regulating the planet's climate.","attribution":"Nature"},
+  {"category":"nature","content":"Bees pollinate roughly a third of the crops humans rely on for food, making them essential to global agriculture despite their small size.","attribution":"Nature"},
+  {"category":"nature","content":"The deepest part of the ocean, the Mariana Trench, plunges nearly eleven kilometers below the surface, deeper than Mount Everest is tall.","attribution":"Nature"},
+  {"category":"nature","content":"Deserts are defined by low precipitation rather than temperature, which is why some of the coldest places on Earth, including parts of Antarctica, are technically deserts.","attribution":"Nature"},
+  {"category":"nature","content":"Mycorrhizal fungi form partnerships with the roots of most plants, helping them absorb water and nutrients in exchange for sugars produced by photosynthesis.","attribution":"Nature"},
+  {"category":"nature","content":"Wolves live and hunt in structured packs, usually led by a breeding pair, and communicate through howls that can be heard for several kilometers.","attribution":"Nature"},
+  {"category":"nature","content":"The Sahara Desert was once a green landscape with lakes and grasslands, before a shift in the Earth's orbit altered rainfall patterns thousands of years ago.","attribution":"Nature"},
+  {"category":"nature","content":"Rivers shape the landscape around them over thousands of years, carving canyons and depositing sediment that forms fertile floodplains.","attribution":"Nature"},
+  {"category":"nature","content":"A rainforest canopy can be so dense that less than two percent of sunlight reaches the forest floor below.","attribution":"Nature"}
+]