refactor(core): extract base32 module, dedupe two RFC 4648 impls

New crates/relicario-core/src/base32.rs hosts encode_rfc4648 +
decode_rfc4648_lenient (case-insensitive, optional padding, whitespace
stripped). Folds inline base32_encode (item.rs:255-275) and
decode_base32_totp (import_lastpass.rs:202-220) into the shared module;
both call sites updated.

- New RelicarioError::InvalidBase32(String) variant for the decoder
  error path
- Module is pub(crate); public API surface unchanged
- Steam alphabet (item_types/totp.rs:13) intentionally separate with
  neighbour comment pointing at crate::base32

Plan B Phase 7 sub-step 1 (DEV-A P2 base32 dedup half).
docs/superpowers/specs/2026-05-04-cli-restructure-design.md.

cargo test --workspace: green
cargo clippy --workspace: silent

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/relicario-core/src/lib.rs

@@ -46,6 +46,8 @@ pub use crypto::{decrypt, derive_master_key, encrypt, KdfParams, VERSION_BYTE};
 pub mod ids;
 pub use ids::{AttachmentId, FieldId, ItemId};
 
+pub(crate) mod base32;
+
 pub mod time;
 pub use time::{now_unix, MonthYear};