Export/Import: ZIP export with scope selection, import with public post merge

Export (export.rs): ZIP archive with auto-chunking at 4GB. Four scopes:
identity only, posts only, posts+identity, everything (posts+key+follows+
profiles+settings). Includes blobs. Manifest JSON tracks metadata.

Import (import.rs): Read ZIP summary without importing (preview).
Import public posts into current identity with new PostIds + original
timestamps. Import as new identity (creates identity subdir from key).
Uses spawn_blocking for ZIP I/O to avoid Send issues with ZipArchive.

Tauri IPC: export_data, import_summary, import_public_posts,
import_as_new_identity commands. IdentityManager.base_dir() getter.

Frontend: Export wizard lightbox with scope radio buttons + output dir.
Import wizard with ZIP path, preview summary, action selection.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/core/src/lib.rs

@@ -4,7 +4,9 @@ pub mod connection;
 pub mod content;
 pub mod crypto;
 pub mod http;
+pub mod export;
 pub mod identity;
+pub mod import;
 pub mod network;
 pub mod node;
 pub mod protocol;