fix(acp): harden session-page connection and localize backend errors

- Session-page connect never triggers download/install; returns
  SdkNotInstalled immediately and prompts the user to install from
  Agent Settings instead
- Binary agents now accept any cached version via
  find_best_cached_binary_for_agent so stale caches still connect
- Bound Initialize handshake with a 60s timeout and convert it to
  AcpError::InitializeTimeout via a sentinel in run_connection
- Spawn background task owns ConnectionManager map insertion and
  removes the entry on exit through an RAII guard that survives
  panics, preventing leaked stale entries
- AcpError gains SdkNotInstalled and InitializeTimeout variants plus
  a stable code() identifier; AcpEvent::Error carries code so the
  frontend can render localized messages by key
- Frontend preflight now runs for all connect sources; error event
  handler switches on code to show translated text for
  initialize_timeout, sdk_not_installed, platform_not_supported,
  process_exited, spawn_failed and download_failed
- Remove ConnectionStatus::Downloading enum variant, all frontend
  branches, and i18n strings; drop obsolete autoLinkFailedTitle,
  autoLinkPreflightFailed, preflightCheckFailedDefault and
  preflightFailedTitle keys across 10 locales
- Add backendErrors.* translations in 10 languages
- Diagnostic logging: always log agent stderr plus binary
  path/size/args/env keys and Initialize timing; gate stdin/stdout
  JSON-RPC tracing behind CODEG_ACP_DEBUG to avoid persisting user
  content into OS log files

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

src-tauri/src/acp/preflight.rs

@@ -299,32 +299,51 @@ async fn check_binary_environment(
     };
     checks.push(platform_check);
 
-    // Check binary cache
+    // Check binary cache.
+    //
+    // Pass as long as *any* cached version is present — the session-page
+    // connect path uses the best cached version via
+    // `find_best_cached_binary_for_agent`, so an older-but-working cache
+    // should still be considered "ready". If the cached version differs
+    // from the registry's recommended version, we note it in the message
+    // but still pass — the Settings page's version-badge flow is the
+    // canonical place to surface "upgrade available".
     if platform_supported {
-        let cache_check = match binary_cache::find_cached_binary_for_agent(agent_type, version, cmd)
-        {
-            Ok(Some(_)) => CheckItem {
-                check_id: "binary_cached".into(),
-                label: "Binary cache".into(),
-                status: CheckStatus::Pass,
-                message: "Binary is cached locally".into(),
-                fixes: vec![],
-            },
-            Ok(None) => CheckItem {
-                check_id: "binary_cached".into(),
-                label: "Binary cache".into(),
-                status: CheckStatus::Warn,
-                message: "Binary not cached yet, will be downloaded on first connection".into(),
-                fixes: vec![],
-            },
-            Err(_) => CheckItem {
-                check_id: "binary_cached".into(),
-                label: "Binary cache".into(),
-                status: CheckStatus::Warn,
-                message: "Cannot determine binary cache path".into(),
-                fixes: vec![],
-            },
-        };
+        let cache_check =
+            match binary_cache::find_best_cached_binary_for_agent(agent_type, cmd) {
+                Ok(Some((_, cached_version))) => {
+                    let message = if cached_version == version {
+                        "Binary is cached locally".to_string()
+                    } else {
+                        format!(
+                            "Binary {cached_version} is cached locally (recommended: {version})"
+                        )
+                    };
+                    CheckItem {
+                        check_id: "binary_cached".into(),
+                        label: "Binary cache".into(),
+                        status: CheckStatus::Pass,
+                        message,
+                        fixes: vec![],
+                    }
+                }
+                Ok(None) => CheckItem {
+                    check_id: "binary_cached".into(),
+                    label: "Binary cache".into(),
+                    status: CheckStatus::Warn,
+                    message:
+                        "Binary is not installed. Download it from Agent Settings before connecting."
+                            .into(),
+                    fixes: vec![],
+                },
+                Err(_) => CheckItem {
+                    check_id: "binary_cached".into(),
+                    label: "Binary cache".into(),
+                    status: CheckStatus::Warn,
+                    message: "Cannot determine binary cache path".into(),
+                    fixes: vec![],
+                },
+            };
         checks.push(cache_check);
     }