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.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]
   )