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/components/conversations/conversation-detail-panel.tsx

@@ -125,7 +125,7 @@ function normalizeErrorMessage(error: unknown): string {
   return String(error)
 }
 
-function isExpectedAutoLinkError(error: unknown): boolean {
+function isExpectedConnectError(error: unknown): boolean {
   if (!error || typeof error !== "object") return false
   return (error as { alerted?: unknown }).alerted === true
 }
@@ -310,8 +310,7 @@ const ConversationTabView = memo(function ConversationTabView({
   useEffect(() => {
     connStatusRef.current = connStatus
   }, [connStatus])
-  const isConnecting =
-    connStatus === "connecting" || connStatus === "downloading"
+  const isConnecting = connStatus === "connecting"
   const connectionModes = useMemo(
     () => conn.modes?.available_modes ?? [],
     [conn.modes?.available_modes]
@@ -756,15 +755,13 @@ const ConversationTabView = memo(function ConversationTabView({
       const s = connStatusRef.current
       const doConnect = () => {
         if (!workingDirForConnection) return
-        connConnect(nextAgentType, workingDirForConnection, undefined, {
-          source: "auto_link",
-        })
+        connConnect(nextAgentType, workingDirForConnection, undefined)
           .then(() => {
             setAgentConnectError(null)
           })
           .catch((e) => {
             setAgentConnectError(normalizeErrorMessage(e))
-            if (!isExpectedAutoLinkError(e)) {
+            if (!isExpectedConnectError(e)) {
               console.error("[ConversationTabView] switch agent:", e)
             }
           })