feat: auto-start local server from REPL

When no server is reachable, the REPL now automatically spawns a
qpq-server child process with dev defaults (--allow-insecure-auth,
matching TLS cert paths). The server is killed on REPL exit via a
Drop guard. Use --no-server to opt out (e.g. for remote servers).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/quicproquo-client/src/main.rs

@@ -70,6 +70,10 @@ struct Args {
     #[arg(long, env = "QPQ_PASSWORD")]
     password: Option<String>,
 
+    /// Do not auto-start a local qpq-server (useful when connecting to a remote server).
+    #[arg(long, env = "QPQ_NO_SERVER")]
+    no_server: bool,
+
     #[command(subcommand)]
     command: Option<Command>,
 }
@@ -301,6 +305,9 @@ enum Command {
         /// OPAQUE password (prompted securely if --username is set but --password is not).
         #[arg(long, env = "QPQ_PASSWORD")]
         password: Option<String>,
+        /// Do not auto-start a local qpq-server.
+        #[arg(long, env = "QPQ_NO_SERVER")]
+        no_server: bool,
     },
 
     /// Interactive 1:1 chat: type to send, incoming messages printed as [peer] <msg>. Ctrl+D to exit.
@@ -350,11 +357,13 @@ async fn main() -> anyhow::Result<()> {
     let state_pw = args.state_password.as_deref();
 
     // Default to REPL when no subcommand is given.
+    let no_server = args.no_server;
     let command = args.command.unwrap_or_else(|| Command::Repl {
         state: args.state,
         server: args.server,
         username: args.username,
         password: args.password,
+        no_server,
     });
 
     match command {
@@ -553,6 +562,7 @@ async fn main() -> anyhow::Result<()> {
             server,
             username,
             password,
+            no_server,
         } => {
             let local = tokio::task::LocalSet::new();
             local
@@ -566,6 +576,7 @@ async fn main() -> anyhow::Result<()> {
                     password.as_deref(),
                     &args.access_token,
                     args.device_id.as_deref(),
+                    no_server,
                 ))
                 .await
         }