feat(walletkit-db): enable persistent storage for WASM builds

[Dzejkop]

Summary

Adds an optional wasm-storage feature to walletkit-db that enables persistent storage backends for WASM builds, building on top of the FFI fixes in #251.

What changed

• walletkit-db/Cargo.toml: Added wasm-storage feature flag gating the sqlite-wasm-vfs dependency
• walletkit-db/src/wasm_storage.rs: New module exposing two persistent VFS backends:
  • install_opfs_sahpool() — OPFS-backed, full durability (Dedicated Worker only)
  • install_relaxed_idb() — IndexedDB-backed, relaxed durability (any context)
• walletkit-db/src/lib.rs: Conditionally includes wasm_storage module when wasm32 + wasm-storage

How it works

By default, sqlite-wasm-rs uses an in-memory VFS that loses all data on page reload. The sqlite-wasm-vfs companion crate provides two persistent alternatives:

	MemoryVFS (default)	OPFS sahpool	IndexedDB relaxed-idb
Storage	RAM	OPFS	IndexedDB
Contexts	All	Dedicated Worker	All
Full durability	✅	✅	❌
Relaxed durability	❌	❌	✅

Usage:

// Call once before opening any database:
walletkit_db::wasm_storage::install_opfs_sahpool(None).await?;
// — or —
walletkit_db::wasm_storage::install_relaxed_idb(None).await?;

// Then open databases as usual — they will be persisted:
let conn = Connection::open(Path::new("app.db"), false)?;

Validation

• ✅ cargo check -p walletkit-db --target wasm32-unknown-unknown (without feature)
• ✅ cargo check -p walletkit-db --target wasm32-unknown-unknown --features wasm-storage (with feature)
• ✅ cargo check -p walletkit-db (native, unchanged)

Note

CI workflow change to add the --features wasm-storage compile check was prepared but couldn't be pushed due to token scope limitations (workflow scope required). The CI diff is:

- name: Run WASM compile check (with wasm-storage)
  run: cargo check -p walletkit-db --target wasm32-unknown-unknown --features wasm-storage

[Dzejkop]

Superseded — taking a different approach: removing the storage feature gate entirely instead of adding a new wasm-storage feature.