Sounds: Japanese-house sound set in the app, layout demo removed

Eleven sounds for events that carry news (ready, unready, everyone ready,
join, leave, kick, new video, others' chat, idle warning, connection
lost/back); none for what the video already says. New voices: koto
(Karplus-Strong, bendable), furin chime, rin bowl, bamboo, hyoshigi,
shoji slide, in the Yo scale on D. Played by SynthSfx at the Settings
volume with per-sound switches and previews. The debug layout mock is
gone.

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

docs/ARCHITECTURE.md

@@ -528,8 +528,7 @@ sideways until an end, then the page).
   of `NetworkInterface.list` (IPv4, no loopback or link-local, private
   ranges first) with the Host port, and a note that friends outside the
   LAN need a forwarded port or a reachable server). Empty strip space is the window
-  drag area; the window buttons sit at its end. Debug builds add a button
-  that opens the layout mock (`screens/demo/`).
+  drag area; the window buttons sit at its end.
 - **Avatars** (`avatar.dart`): `StatusAvatar` draws a ring outside the
   picture with a gap in the background colour (both scale with the size):
   green ready, amber buffering (with a spinner), grey not ready, dimmed
@@ -590,13 +589,19 @@ sideways until an end, then the page).
   Jellyfin & integrations (linked servers, AnimeFlow's `/me` via
   `AnimeFlowClient.fromAccounts`, auto-advance), notifications & sounds
   (what pops up; no switches), updates, shortcuts, about.
-- **Sounds** (`sfx/`): the coordinator plays an `SfxCue` per feed event
-  through `SfxService` (the app still uses `NoopSfxService`). The set is
-  synthesized, not sampled: `synth.dart` renders notes (sine/bell/pluck
-  voices, soft attacks, low-pass, short room tail) to quiet WAVs,
-  `sound_bank.dart` defines every sound in D major pentatonic, and
-  `SynthSfx` plays them on a small pool of audio-only mpv players. The
-  debug layout demo's Notifications & sounds page plays each one.
+- **Sounds** (`sfx/`): the coordinator plays an `SfxCue` for ready,
+  unready, force-ready, join, leave, kick, swap and others' chat (feed
+  events), the idle warning, and connection lost / back (once per drop).
+  Nothing for what the video already says (play, pause, seeks, the end).
+  The set is synthesized, not sampled: `synth.dart` renders Japanese-house
+  voices (Karplus-Strong koto with bends, furin chime, rin bowl, bamboo,
+  hyoshigi clappers, a shoji slide of filtered noise) through a low-pass
+  and a short room to quiet WAVs; `sound_bank.dart` voices each event in
+  the Yo scale on D. `SynthSfx` renders a sound on first use into the temp
+  dir and plays it on a pool of three audio-only mpv players, at
+  `AppSettings.soundVolume` (square curve) unless switched off in
+  `soundsOff`. Settings → Notifications & sounds has the volume and a
+  preview and switch per sound.
 - **Player** (`session_player.dart`): `SessionPlayer` holds the one
   `Video` of the current player (a new `GlobalKey` per player generation).
   The shell keeps it in one layer of the body at one place in the element