feat: remember last nickname + sanitize chat input

Remember last nickname: a new serde-default AppConfig.username field is
pre-filled into the launch-screen nickname field, and saved when a room is
joined or created (i.e. when the name is actually used), so it carries across
launches.

Sanitize chat: a pure sanitize_chat() drops control characters (ANSI escapes,
NUL, stray CR/LF/TAB), collapses whitespace runs to single spaces, trims, and
caps length (2000 chars). Applied to our outgoing text on submit AND to incoming
peer messages on receive — peer content is untrusted, so the sender's name and
text are both sanitized before display; empty-after-sanitize messages are
dropped. Unit tests for sanitize_chat (control/whitespace/unicode/empty + length
cap) and a config backward-compat assertion for username.

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

src/config.rs

@@ -44,6 +44,10 @@ fn default_volume() -> f32 {
     1.0
 }
 
+fn default_username() -> String {
+    "Peer".to_string()
+}
+
 fn default_participants_width() -> f32 {
     540.0
 }
@@ -54,6 +58,9 @@ fn default_chat_height() -> f32 {
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct AppConfig {
+    /// Last nickname used to join/create a room; pre-filled on the launch screen.
+    #[serde(default = "default_username")]
+    pub username: String,
     pub input_device: String,
     pub output_device: String,
     pub noise_gate_threshold: f32,
@@ -98,6 +105,7 @@ pub struct AppConfig {
 impl Default for AppConfig {
     fn default() -> Self {
         Self {
+            username: default_username(),
             input_device: "".to_string(),
             output_device: "".to_string(),
             noise_gate_threshold: 0.01,
@@ -176,6 +184,8 @@ mod tests {
         // Configs predating the draggable dividers must load the default sizes.
         assert_eq!(deserialized.participants_width, 540.0);
         assert_eq!(deserialized.chat_height, 180.0);
+        // Configs predating the remembered username load the default nickname.
+        assert_eq!(deserialized.username, "Peer");
         assert!(deserialized.custom_sound_self_join.is_none());
         assert!(deserialized.custom_sound_peer_join.is_none());
         assert!(deserialized.custom_sound_peer_leave.is_none());