Restructure into themed screens with flag card backs, real font, flip animation and sound

- Replace the bogus DroidSansJapanese.ttf (an HTML page) with M PLUS Rounded 1c,
  loaded with LoadFontEx and explicit kana codepoints
- Theme struct per language with flag-based card backs (Hinomaru, Taegukgi, Trairong)
- Card art rendered to textures so the flip squashes the whole card
- Synthesized flip/match/mismatch/win/click sounds (new audio package)
- Shared ui package for text and buttons; Screen enum replaces i32 ids
- Kana <-> romaji pairs drawn from all 46 basic hiragana

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/audio/audio.odin

@@ -0,0 +1,135 @@
+package audio
+
+import "core:math"
+import "core:math/rand"
+import raylib "vendor:raylib"
+
+// All sounds are synthesized at startup, so no audio files are needed
+
+SAMPLE_RATE :: 44100
+
+Sound_Effect :: enum {
+    FLIP,
+    MATCH,
+    MISMATCH,
+    WIN,
+    CLICK,
+}
+
+sounds: [Sound_Effect]raylib.Sound
+ready: bool
+
+@(private)
+Tone :: struct {
+    start:    f32, // seconds
+    freq:     f32, // Hz
+    duration: f32, // seconds
+    amp:      f32,
+}
+
+// Sine with a little 2nd/3rd harmonic, soft attack and exponential decay: a small bell
+@(private)
+add_tone :: proc(samples: []f32, tone: Tone) {
+    first := int(tone.start * SAMPLE_RATE)
+    count := int(tone.duration * SAMPLE_RATE)
+    attack: f32 = 0.006
+    for i in 0 ..< count {
+        idx := first + i
+        if idx >= len(samples) { break }
+        t := f32(i) / SAMPLE_RATE
+        env := min(t / attack, 1) * math.exp(-5 * t / tone.duration)
+        phase := 2 * math.PI * tone.freq * t
+        s := math.sin(phase) + 0.25 * math.sin(2 * phase) + 0.08 * math.sin(3 * phase)
+        samples[idx] += tone.amp * env * s
+    }
+}
+
+// Card "snap": low-passed noise with a fast decay over a short low thump
+@(private)
+add_flip :: proc(samples: []f32) {
+    lp: f32 = 0
+    for i in 0 ..< len(samples) {
+        t := f32(i) / SAMPLE_RATE
+        noise := rand.float32_range(-1, 1)
+        lp += 0.35 * (noise - lp)
+        swish := lp * math.exp(-t * 55) * min(t / 0.004, 1)
+        thump := math.sin(2 * math.PI * 150 * t) * math.exp(-t * 70)
+        samples[i] += 0.55 * swish + 0.35 * thump
+    }
+}
+
+@(private)
+make_sound :: proc(samples: []f32) -> raylib.Sound {
+    pcm := make([]i16, len(samples))
+    defer delete(pcm)
+    for s, i in samples {
+        pcm[i] = i16(math.clamp(s, -1, 1) * 32000)
+    }
+    wave := raylib.Wave{
+        frameCount = u32(len(pcm)),
+        sampleRate = SAMPLE_RATE,
+        sampleSize = 16,
+        channels   = 1,
+        data       = raw_data(pcm),
+    }
+    // LoadSoundFromWave copies the samples into its own buffer
+    return raylib.LoadSoundFromWave(wave)
+}
+
+@(private)
+build :: proc(duration: f32, tones: []Tone, flip := false) -> raylib.Sound {
+    samples := make([]f32, int(duration * SAMPLE_RATE))
+    defer delete(samples)
+    if flip { add_flip(samples) }
+    for tone in tones { add_tone(samples, tone) }
+    return make_sound(samples)
+}
+
+// Call after raylib.InitAudioDevice
+load :: proc() {
+    if !raylib.IsAudioDeviceReady() { return }
+
+    E5 :: 659.25
+    A5 :: 880.00
+    B5 :: 987.77
+    C5 :: 523.25
+    G5 :: 783.99
+    C6 :: 1046.50
+    E6 :: 1318.51
+
+    sounds[.FLIP] = build(0.12, {}, flip = true)
+    sounds[.MATCH] = build(0.6, {
+        {start = 0.00, freq = E5, duration = 0.35, amp = 0.30},
+        {start = 0.09, freq = B5, duration = 0.50, amp = 0.28},
+    })
+    sounds[.MISMATCH] = build(0.45, {
+        {start = 0.00, freq = 330, duration = 0.20, amp = 0.20},
+        {start = 0.11, freq = 262, duration = 0.30, amp = 0.20},
+    })
+    sounds[.WIN] = build(1.3, {
+        {start = 0.00, freq = C5, duration = 0.40, amp = 0.18},
+        {start = 0.11, freq = E5, duration = 0.40, amp = 0.18},
+        {start = 0.22, freq = G5, duration = 0.40, amp = 0.18},
+        {start = 0.33, freq = C6, duration = 0.90, amp = 0.2},
+        {start = 0.33, freq = E6, duration = 0.90, amp = 0.07},
+    })
+    sounds[.CLICK] = build(0.08, {{start = 0, freq = A5, duration = 0.06, amp = 0.15}})
+
+    raylib.SetSoundVolume(sounds[.FLIP], 0.7)
+    ready = true
+}
+
+unload :: proc() {
+    if !ready { return }
+    for s in sounds { raylib.UnloadSound(s) }
+    ready = false
+}
+
+play :: proc(effect: Sound_Effect) {
+    if !ready { return }
+    if effect == .FLIP {
+        // Slight pitch variation so repeated flips don't sound identical
+        raylib.SetSoundPitch(sounds[.FLIP], rand.float32_range(0.9, 1.12))
+    }
+    raylib.PlaySound(sounds[effect])
+}