Make per-peer in-call volume continuous and persistent

The per-peer volume slider had no .step(), so iced's default step of
1.0 on a 0.0..=2.0 range meant it could only snap to 0%, 100%, or
200% — it felt like hard-left/hard-right only. Add .step(0.01) for
smooth 1%-increment control (matching the Pan slider below it, which
already set its own step).

Also persist per-peer volume across sessions, mirroring peer_pan/peer_eq:
- new AppConfig.peer_volume map (keyed by peer id string, serde default
  for back-compat; never sent over the wire)
- replace the in-memory peer_volumes map with config-backed storage via
  a new set_peer_volume_config helper (clamps to range, drops at-unity
  entries so the config stays tidy)
- replay saved volumes to core on startup alongside pan/eq
- the slider writes to disk on release (AppMessage::PersistConfig)

+1 unit test for the config helper; +1 config back-compat assertion.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.rs

@@ -255,6 +255,10 @@ pub struct AppConfig {
     /// keyed by peer node id string. Local preference only.
     #[serde(default)]
     pub peer_pan: HashMap<String, f32>,
+    /// Per-peer listener-side volume/gain (`1.0` = unity), keyed by peer node id
+    /// string. Local preference only; never sent to peers. Absent entry = unity.
+    #[serde(default)]
+    pub peer_volume: HashMap<String, f32>,
     /// Focused app-local keyboard shortcuts.
     #[serde(default)]
     pub hotkeys: crate::hotkeys::HotkeyMap,
@@ -317,6 +321,7 @@ impl Default for AppConfig {
             recents: Vec::new(),
             peer_eq: HashMap::new(),
             peer_pan: HashMap::new(),
+            peer_volume: HashMap::new(),
             hotkeys: crate::hotkeys::HotkeyMap::default(),
             window_width: default_window_width(),
             window_height: default_window_height(),
@@ -457,6 +462,7 @@ mod tests {
         // shortcut settings.
         assert!(deserialized.peer_eq.is_empty());
         assert!(deserialized.peer_pan.is_empty());
+        assert!(deserialized.peer_volume.is_empty());
         assert_eq!(
             crate::hotkeys::format_binding(
                 deserialized