fix(persistence): atomic + owner-only settings/address-book writes

settings.json and addressbook.json were written in place with a bare ofstream —
a crash or power loss mid-write truncated the file, and on the next launch the
parse failure silently reset every preference (hidden/favorite addresses, labels,
pool workers, language, theme, lite-server list) because the next save overwrote
the corrupt file with defaults.

Add Platform::writeFileAtomically() (temp file -> fsync -> atomic rename; dir
fsync on POSIX, MoveFileEx on Windows; optional owner-only 0600) and route both
saves through it. On a parse failure, quarantine the unreadable settings file to
settings.json.corrupt-<ts> instead of clobbering it.

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

src/util/platform.h

@@ -74,6 +74,26 @@ public:
      * @return true if successful or file didn't exist
      */
     static bool deleteFile(const std::string& path);
+
+    /**
+     * @brief Write a file atomically and durably.
+     *
+     * Writes @p content to `<path>.tmp`, flushes it to stable storage
+     * (fsync / FlushFileBuffers), then atomically renames it over @p path
+     * (POSIX rename / Win32 MoveFileEx with REPLACE_EXISTING). A crash or power
+     * loss at any point leaves either the old file intact or the fully-written
+     * new one — never a truncated/corrupt file. The parent directory is created
+     * if missing, and on POSIX it is fsync'd so the rename itself is durable.
+     *
+     * @param path Destination file path.
+     * @param content Bytes to write.
+     * @param restrictPermissions When true, create the file owner-only (0600 on
+     *        POSIX) before the rename so it is never briefly world-readable.
+     * @return true on success; on failure any pre-existing file is left untouched.
+     */
+    static bool writeFileAtomically(const std::string& path,
+                                    const std::string& content,
+                                    bool restrictPermissions = false);
     
     /**
      * @brief Get the directory containing the executable