Fix bundle lookup: normalize fingerprints, handle 404 gracefully

Server: normalize fingerprints by stripping colons and lowercasing
before storing/looking up in sled. Adds tracing for register/lookup.

Client: check HTTP status before parsing JSON response body.
Shows clear error when user is not registered instead of parse error.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

warzone/crates/warzone-client/src/net.rs

@@ -59,7 +59,7 @@ impl ServerClient {
 
     /// Fetch a user's pre-key bundle from the server.
     pub async fn fetch_bundle(&self, fingerprint: &str) -> Result<PreKeyBundle> {
-        let resp: BundleResponse = self
+        let response = self
             .client
             .get(format!(
                 "{}/v1/keys/{}",
@@ -67,7 +67,17 @@ impl ServerClient {
             ))
             .send()
             .await
-            .context("failed to fetch bundle")?
+            .context("failed to fetch bundle")?;
+
+        if !response.status().is_success() {
+            anyhow::bail!(
+                "server returned {} — user {} may not be registered",
+                response.status(),
+                fingerprint
+            );
+        }
+
+        let resp: BundleResponse = response
             .json()
             .await
             .context("failed to parse bundle response")?;