fix(startup): surface filesystem failures and verify Sapling param integrity

Three verified daemon-startup edge-case fixes centered on the config/params
filesystem path:

- F7: new non-throwing Platform::ensureDirectory(dir, outError) with one
  consistent "Cannot create <dir>: <reason>. Check permissions / free space."
  message. Replaces the unchecked/throwing create_directories sites at main.cpp
  (pre-init: log + Windows MessageBox + return 1), connection.cpp's
  autoDetectConfig (was the *throwing* overload -- could raise an uncaught
  filesystem_error through its callers; now sets the new
  ConnectionConfig::dir_error), and both app.cpp daemon-dir sites (surface via
  daemon_status_ + return false). The primary connect path (app_network.cpp)
  checks dir_error and shows it instead of mislabelling it "waiting for config".
  embedded_resources.cpp already checked its error_code, so it is left as-is.

- F6: verifySaplingParams() now hash-verifies each param against its pinned
  canonical SHA-256 (source of truth: scripts/build-lite-backend-artifact.sh)
  instead of only checking existence, so a truncated / corrupt-but-present param
  is rejected up front rather than failing later on a shielded operation. A
  <params_dir>/.sapling_verified marker keyed on size:mtime avoids re-hashing
  ~48MB on every startup. Logic extracted to the injectable, unit-testable
  verifySaplingParamsIn(dir, digests); reuses util::sha256Hex (no new hash impl).

- F5: startEmbeddedDaemon() now checks extractEmbeddedResources()'s return and
  the previously-dropped copy_file error_code in the daemon-binary fallback loop,
  aborting with a clear status (sb_daemon_extract_failed / sb_daemon_files_failed)
  instead of failing opaquely at spawn. An absent source file stays non-fatal.

Adds testPlatformEnsureDirectory and testVerifySaplingParams to test_phase4.cpp.
i18n keys added to i18n.cpp (English source of truth); the res/lang/*.json
back-fill via add_missing_translations.py is deferred to a single run at the end
of the batch. Progress tracked in docs/daemon-startup-hardening.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/rpc/connection.h

@@ -5,6 +5,8 @@
 #pragma once
 
 #include <string>
+#include <vector>
+#include <utility>
 
 namespace dragonx {
 namespace rpc {
@@ -28,6 +30,9 @@ struct ConnectionConfig {
     bool use_embedded = true;
     bool use_tls = false;
     AuthSource auth_source = AuthSource::Missing;
+    // Non-empty when autoDetectConfig() could not create the data directory; callers
+    // should surface it and abort the connect rather than proceeding blindly.
+    std::string dir_error;
 };
 
 /**
@@ -69,6 +74,14 @@ public:
      */
     static bool verifySaplingParams();
 
+    // Verify the Sapling params in `dir` against a { filename, expected-sha256-hex } list.
+    // Exposed with an injectable dir + digest list so the integrity + marker-cache logic is
+    // unit-testable without the real ~48MB params; verifySaplingParams() calls it with the
+    // pinned production digests and getSaplingParamsDir().
+    static bool verifySaplingParamsIn(
+        const std::string& dir,
+        const std::vector<std::pair<std::string, std::string>>& digests);
+
     /**
      * @brief Get the Sapling params directory
      */