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/science.json b/data/packs/science.json
new file mode 100644
index 0000000..f105138
--- /dev/null
+++ b/data/packs/science.json
@@ -0,0 +1,17 @@
+[
+  {"category":"science","content":"Photosynthesis is the process by which plants convert light energy into chemical energy, using carbon dioxide and water to produce glucose and release oxygen as a byproduct.","attribution":"Science"},
+  {"category":"science","content":"Newton's first law of motion states that an object at rest stays at rest, and an object in motion stays in motion, unless acted on by an outside force.","attribution":"Science"},
+  {"category":"science","content":"DNA carries the genetic instructions for every living organism, encoded in a sequence of four chemical bases that pair together to form the famous double helix shape.","attribution":"Science"},
+  {"category":"science","content":"The theory of plate tectonics explains how the Earth's outer shell is broken into large plates that slowly drift over the mantle, causing earthquakes, volcanoes, and mountain formation.","attribution":"Science"},
+  {"category":"science","content":"Water is one of the few substances that becomes less dense as it freezes, which is why ice floats on top of liquid water instead of sinking to the bottom.","attribution":"Science"},
+  {"category":"science","content":"A black hole forms when a massive star collapses under its own gravity, packing so much mass into so small a space that not even light can escape its pull.","attribution":"Science"},
+  {"category":"science","content":"Vaccines work by training the immune system to recognize a weakened or partial version of a pathogen, so the body can respond quickly if it ever encounters the real threat.","attribution":"Science"},
+  {"category":"science","content":"The periodic table arranges chemical elements by increasing atomic number, grouping elements with similar properties into the same columns.","attribution":"Science"},
+  {"category":"science","content":"Sound travels as a wave of pressure through a medium such as air or water, and it cannot travel through the vacuum of space, since there are no particles to carry the vibration.","attribution":"Science"},
+  {"category":"science","content":"Evolution by natural selection describes how organisms with traits better suited to their environment tend to survive and reproduce more successfully, gradually shaping a population over many generations.","attribution":"Science, after Charles Darwin"},
+  {"category":"science","content":"The speed of light in a vacuum is approximately two hundred ninety nine thousand, seven hundred ninety two kilometers per second, and nothing with mass can reach or exceed it.","attribution":"Science"},
+  {"category":"science","content":"Mitochondria convert nutrients into a usable form of energy for the cell, which is why biology textbooks often call them the powerhouse of the cell.","attribution":"Science"},
+  {"category":"science","content":"The human body contains roughly thirty seven trillion cells, each one specialized to carry out one of hundreds of different jobs inside the body.","attribution":"Science"},
+  {"category":"science","content":"Marie Curie remains the only person to win Nobel Prizes in two different sciences, physics and chemistry, for her pioneering research into radioactivity.","attribution":"Science"},
+  {"category":"science","content":"Genetics research has shown that humans share about ninety-nine percent of their DNA with chimpanzees, the closest living relative to our species.","attribution":"Science"}
+]