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/hooks/use-connection-lifecycle.ts

@@ -33,7 +33,7 @@ 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
 }
@@ -77,12 +77,10 @@ export function useConnectionLifecycle({
   const modeLoading =
     !hasCachedSelectors &&
     (status === "connecting" ||
-      status === "downloading" ||
       (isInteractiveStatus && !effectiveSelectorsReady))
   const configOptionsLoading =
     !hasCachedSelectors &&
     (status === "connecting" ||
-      status === "downloading" ||
       (isInteractiveStatus && !effectiveSelectorsReady))
   // Gate for send button: block until the backend session is fully
   // initialized (selectorsReady from the real backend event, not cache).
@@ -141,9 +139,7 @@ export function useConnectionLifecycle({
     const s = statusRef.current
     if (!s || s === "disconnected" || s === "error") {
       connConnectRef
-        .current(agentTypeRef.current, workingDir, sessionIdRef.current, {
-          source: "auto_link",
-        })
+        .current(agentTypeRef.current, workingDir, sessionIdRef.current)
         .then(() => {
           if (!cancelled) {
             setLastAutoConnectError(null)
@@ -157,7 +153,7 @@ export function useConnectionLifecycle({
               message: normalizeErrorMessage(e),
             })
           }
-          if (!isExpectedAutoLinkError(e)) {
+          if (!isExpectedConnectError(e)) {
             console.error("[ConnLifecycle] auto-connect:", e)
           }
         })
@@ -170,7 +166,7 @@ export function useConnectionLifecycle({
   // Manage task status for connection progress
   const taskIdRef = useRef<string | null>(null)
   useEffect(() => {
-    if (status === "connecting" || status === "downloading") {
+    if (status === "connecting") {
       if (!taskIdRef.current) {
         const id = `acp-connect-${Date.now()}`
         taskIdRef.current = id
@@ -271,10 +267,8 @@ export function useConnectionLifecycle({
     touchActivity(contextKey)
     if (!status || status === "disconnected" || status === "error") {
       setLastAutoConnectError(null)
-      connConnect(agentType, workingDir, sessionId, {
-        source: "auto_link",
-      }).catch((e: unknown) => {
-        if (!isExpectedAutoLinkError(e)) {
+      connConnect(agentType, workingDir, sessionId).catch((e: unknown) => {
+        if (!isExpectedConnectError(e)) {
           console.error("[ConnLifecycle] connect:", e)
         }
       })

src/hooks/use-connection.ts

@@ -6,7 +6,6 @@ import {
   useConnectionStore,
   getCachedSelectors,
   type ConnectionState,
-  type ConnectOptions,
   type LiveMessage,
   type PendingPermission,
   type PendingQuestion,
@@ -45,8 +44,7 @@ export interface UseConnectionReturn {
   connect: (
     agentType: AgentType,
     workingDir?: string,
-    sessionId?: string,
-    options?: ConnectOptions
+    sessionId?: string
   ) => Promise<void>
   disconnect: () => Promise<void>
   sendPrompt: (blocks: PromptInputBlock[]) => Promise<void>
@@ -96,12 +94,8 @@ export function useConnection(contextKey: string): UseConnectionReturn {
   const error = connection?.error ?? null
 
   const connect = useCallback(
-    (
-      agentType: AgentType,
-      workingDir?: string,
-      sessionId?: string,
-      options?: ConnectOptions
-    ) => actions.connect(contextKey, agentType, workingDir, sessionId, options),
+    (agentType: AgentType, workingDir?: string, sessionId?: string) =>
+      actions.connect(contextKey, agentType, workingDir, sessionId),
     [actions, contextKey]
   )