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/error.rs

@@ -123,6 +123,12 @@ pub enum RelicarioError {
     /// Recovery QR generation or parsing failed.
     #[error("recovery QR: {0}")]
     RecoveryQr(String),
+
+    /// Base32 decoding failed (non-alphabet character or other malformed
+    /// input). Emitted by [`crate::base32::decode_rfc4648_lenient`] and any
+    /// typed wrappers that delegate to it.
+    #[error("invalid base32: {0}")]
+    InvalidBase32(String),
 }
 
 /// Crate-wide result alias, reducing boilerplate in function signatures.