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/binary_cache.rs

@@ -133,6 +133,34 @@ pub fn detect_installed_version(
     installed_version_for_agent(agent_type, cmd_name)
 }
 
+/// Return the best cached binary across all installed versions.
+///
+/// This returns the path + version label of the highest semver-ish
+/// version cached on disk, regardless of what the registry considers
+/// the "recommended" version. The session-page connect path uses this
+/// to tolerate older-but-still-usable cached binaries (e.g. the user
+/// hasn't upgraded yet) — the Settings page will continue to surface
+/// an "upgrade available" hint via the separate version-badge path.
+///
+/// Returns Ok(None) when no usable binary is cached.
+pub fn find_best_cached_binary_for_agent(
+    agent_type: AgentType,
+    cmd_name: &str,
+) -> Result<Option<(PathBuf, String)>, AcpError> {
+    let agent_id = agent_cache_key(agent_type);
+    let mut versions = installed_version_labels(&agent_id, cmd_name)?;
+    if versions.is_empty() {
+        return Ok(None);
+    }
+    versions.sort_by(|a, b| version_cmp(a, b));
+    while let Some(version) = versions.pop() {
+        if let Some(path) = installed_binary_path(&agent_id, &version, cmd_name) {
+            return Ok(Some((path, version)));
+        }
+    }
+    Ok(None)
+}
+
 fn version_cmp(a: &str, b: &str) -> std::cmp::Ordering {
     let mut a_parts = parse_version_parts(a);
     let mut b_parts = parse_version_parts(b);