improved web ui, upload is sorta better, 5sec advertise btn

app/README.md

@@ -25,9 +25,21 @@ Pairs with the firmware in `../baby_mobile_v2/baby_mobile_v2.ino`.
 | | UUID | Use |
 |---|---|---|
 | Service | `12340001-…` | advertised |
-| Cmd (write) | `12340002-…` | `0x02 <slot>` start upload · `0x03` finish · `0x04 <slot>` play · `0x05` stop · `0x06` list · `0x07 <slot>` delete |
-| Data (write-without-response) | `12340003-…` | raw WAV bytes, chunked to 180 B |
-| Status (notify) | `12340004-…` | tagged by byte 0: `0x00…` upload byte count (uint32 BE) · `0x80\|idx, bits, kHz, secs` list entry · `0xFF, n` list end · `0xD0, idx, ok` delete reply |
+| Cmd (write) | `12340002-…` | `0x02 <slot>` start upload · `0x03` finish · `0x04 <slot>` play · `0x05` stop · `0x06` list · `0x07 <slot>` delete · `0x08 <pct>` set volume (persisted) · `0x09` get volume |
+| Data (write-without-response) | `12340003-…` | raw WAV bytes, chunked to MTU−3 |
+| Status (notify) | `12340004-…` | tagged by byte 0: `0x00…` upload byte count (uint32 BE) · `0x80\|idx, bits, kHz, secs` list entry · `0xC0, pct` volume reply · `0xD0, idx, ok` delete reply · `0xFF, n` list end |
+
+### Volume
+
+- **Global volume** is a master gain on the device (Q8 fixed-point, applied to all
+  playback). It's persisted to LittleFS (`/volume`) so it survives reboots. The
+  app reads it on connect (`0x09`) and sets it via the Playback-volume slider
+  (`0x08`). Lowering it reduces the PWM swing → less amp output → less supply
+  current, which mitigates the regulator brown-out at high volume.
+- **Per-file level** is baked into the WAV in the browser *before upload* (the
+  "This file's level" slider scales the samples). Effective loudness =
+  global × per-file. Note: attenuating an 8-bit clip costs dynamic range, so use
+  16-bit if you plan to run a file much quieter than the rest.
 
 ## Running it