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/layout/status-bar-connection.tsx

@@ -19,7 +19,6 @@ import { cn } from "@/lib/utils"
 type ConnectionStatusLabelKey =
   | "connected"
   | "connecting"
-  | "downloading"
   | "prompting"
   | "error"
 
@@ -32,10 +31,6 @@ const STATUS_STYLE: Record<
     className: "opacity-100 animate-pulse",
     labelKey: "connecting",
   },
-  downloading: {
-    className: "opacity-100 animate-pulse",
-    labelKey: "downloading",
-  },
   prompting: {
     className: "opacity-100 animate-pulse",
     labelKey: "prompting",