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/i18n/messages/ja.json

@@ -851,7 +851,6 @@
       "connection": {
         "connected": "接続済み",
         "connecting": "接続中...",
-        "downloading": "ダウンロード中...",
         "prompting": "応答中...",
         "error": "接続エラー",
         "disconnected": "未接続",
@@ -1422,11 +1421,13 @@
           "unavailable": "{agent} は現在のプラットフォームでは利用できません。",
           "sdkMissing": "{agent} SDK がインストールされていません"
         },
+        "backendErrors": {
+          "initializeTimeout": "{agent} の接続ハンドシェイクがタイムアウトしました（60 秒以内に応答なし）。設定を開いてエージェントとネットワーク設定を確認してください。",
+          "processExited": "{agent} プロセスが予期せず終了しました。",
+          "spawnFailed": "{agent} の起動に失敗しました: {message}",
+          "downloadFailed": "{agent} のダウンロードに失敗しました: {message}"
+        },
         "unableReadAgentConfig": "エージェント設定を読み取れません: {message}",
-        "autoLinkFailedTitle": "{agent} の自動リンクに失敗しました",
-        "preflightCheckFailedDefault": "事前チェックに失敗しました。エージェント設定を確認してください。",
-        "preflightFailedTitle": "{agent} の事前チェックに失敗しました",
-        "autoLinkPreflightFailed": "自動リンクの事前チェックに失敗しました: {message}",
         "connectFailedTitle": "{agent} の接続に失敗しました",
         "toolFallbackTitle": "ツール",
         "eventErrorTitle": "エージェントエラー",