W16 custom backgrounds: core + render layer (Settings UI pending)

Pure src/background.rs (process_background downscale→PNG, scrim_color; 5 tests),
AppConfig.background/background_dim + background_path(), cached AppState.background_image,
PickBackgroundFile/BackgroundFilePicked/RemoveBackground/SetBackgroundDim handlers,
view_with_background stack(image Cover→scrim→ui) + transparent screen roots.
Lib builds clean. Remaining: Settings UI controls + full build/clippy/test pass.

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

src/config.rs

@@ -106,6 +106,10 @@ fn default_true() -> bool {
     true
 }
 
+fn default_background_dim() -> f32 {
+    crate::background::DEFAULT_DIM
+}
+
 fn default_volume() -> f32 {
     1.0
 }
@@ -185,6 +189,16 @@ pub struct AppConfig {
     /// Our chosen avatar (W4): monogram fallback or a bundled preset.
     #[serde(default)]
     pub avatar: crate::avatar::Avatar,
+    /// Custom UI background image (W16): path to the downscaled PNG we wrote into
+    /// the config dir (see `background_path`). `None` = use the theme background.
+    /// Local-only; never sent to peers.
+    #[serde(default)]
+    pub background: Option<String>,
+    /// Scrim strength drawn over the custom background for legibility (0.0 = image
+    /// at full strength, 1.0 = fully hidden behind the theme base). See
+    /// `crate::background::scrim_color`.
+    #[serde(default = "default_background_dim")]
+    pub background_dim: f32,
     /// What a call recording captures (mixed / per-peer stems / both).
     #[serde(default)]
     pub recording_mode: RecordingMode,
@@ -280,6 +294,8 @@ impl Default for AppConfig {
             room_layout: RoomLayout::default(),
             theme: AppTheme::default(),
             avatar: crate::avatar::Avatar::default(),
+            background: None,
+            background_dim: default_background_dim(),
             recording_mode: RecordingMode::default(),
             custom_sound_self_join: None,
             custom_sound_peer_join: None,
@@ -348,6 +364,17 @@ impl AppConfig {
         })
     }
 
+    /// Path the processed custom-background PNG (W16) is written to, alongside
+    /// `config.json` in the app config dir. We store our own downscaled copy here
+    /// (rather than base64 in the config) so the JSON stays small.
+    pub fn background_path() -> Option<PathBuf> {
+        dirs::config_dir().map(|mut p| {
+            p.push("peerspeak");
+            p.push("background.png");
+            p
+        })
+    }
+
     pub fn load() -> Self {
         if let Some(path) = Self::config_path()
             && let Ok(contents) = fs::read_to_string(&path)