Rework avatars.

Now we have a concept of an avatar at the server, realm, and scene level
and we have the groundwork for a realm store. New uesrs no longer props,
they get a default avatar. New system supports gender
{male,female,neutral} and {child,adult}.

crates/chattyness-shared/src/config.rs

@@ -16,6 +16,8 @@ pub struct AppConfig {
     pub websocket: WebSocketConfig,
     /// Stale member cleanup configuration.
     pub cleanup: CleanupConfig,
+    /// Signup form configuration.
+    pub signup: SignupConfig,
 }
 
 /// WebSocket configuration.
@@ -40,11 +42,62 @@ pub struct CleanupConfig {
     pub clear_on_startup: bool,
 }
 
+/// Signup form configuration.
+/// Controls what fields are shown during user registration.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(default)]
+pub struct SignupConfig {
+    /// Whether to show birthday field during signup.
+    /// None = don't ask, Some("ask") = show field
+    pub birthday: Option<SignupField>,
+    /// How to determine user's age category.
+    pub age: AgeConfig,
+    /// How to determine user's gender preference.
+    pub gender: GenderConfig,
+}
+
+/// Signup field option.
+#[derive(Debug, Clone, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum SignupField {
+    /// Show the field during signup
+    Ask,
+}
+
+/// Age category configuration.
+#[derive(Debug, Clone, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum AgeConfig {
+    /// Ask user to select their age category
+    Ask,
+    /// Infer age category from birthday (requires birthday = "ask")
+    Infer,
+    /// Default to adult without asking
+    DefaultAdult,
+    /// Default to child without asking
+    DefaultChild,
+}
+
+/// Gender preference configuration.
+#[derive(Debug, Clone, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum GenderConfig {
+    /// Ask user to select their gender preference
+    Ask,
+    /// Default to neutral without asking
+    DefaultNeutral,
+    /// Default to male without asking
+    DefaultMale,
+    /// Default to female without asking
+    DefaultFemale,
+}
+
 impl Default for AppConfig {
     fn default() -> Self {
         Self {
             websocket: WebSocketConfig::default(),
             cleanup: CleanupConfig::default(),
+            signup: SignupConfig::default(),
         }
     }
 }
@@ -68,6 +121,28 @@ impl Default for CleanupConfig {
     }
 }
 
+impl Default for SignupConfig {
+    fn default() -> Self {
+        Self {
+            birthday: None,
+            age: AgeConfig::default(),
+            gender: GenderConfig::default(),
+        }
+    }
+}
+
+impl Default for AgeConfig {
+    fn default() -> Self {
+        Self::DefaultAdult
+    }
+}
+
+impl Default for GenderConfig {
+    fn default() -> Self {
+        Self::DefaultNeutral
+    }
+}
+
 impl AppConfig {
     /// Load configuration from a TOML file.
     ///