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/lib/types.ts

@@ -256,7 +256,6 @@ export const AGENT_COLORS: Record<AgentType, string> = {
 // ACP connection status (matches Rust ConnectionStatus)
 export type ConnectionStatus =
   | "connecting"
-  | "downloading"
   | "connected"
   | "prompting"
   | "disconnected"
@@ -442,7 +441,14 @@ export type AcpEvent =
       connection_id: string
       status: ConnectionStatus
     }
-  | { type: "error"; connection_id: string; message: string }
+  | {
+      type: "error"
+      connection_id: string
+      message: string
+      agent_type: string
+      /** Stable backend error identifier for localization (e.g. "initialize_timeout"). */
+      code: string | null
+    }
   | {
       type: "available_commands"
       connection_id: string