feat: resolve Pinia store auto-imports

[BartWaardenburg]

Summary

• Adds a built-in Pinia plugin that activates on @pinia/nuxt.
• Resolves direct stores/ and app/stores/ value exports named use<Name>Store as convention auto-import providers.
• Keeps nested stores and custom storesDirs outside the default detector, with tests documenting that boundary.

Issue

Closes #740

Verification

• cargo fmt --all -- --check
• cargo check --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-targets
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps --document-private-items
• target/debug/fallow audit --format json --quiet
• Synthetic Nuxt+Pinia old-vs-new repro: origin/main reports stores/user.ts; this branch keeps stores/user.ts and useUserStore alive while still reporting the unused helper export.
• Public Nuxt+Pinia smoke on BabluMia/Nuxt-Pinia: no crash and no regression on direct store reporting. This repo imports its stores explicitly, so it is not used as the behavioral delta proof.
• Real-world fixtures: zod, preact, and vite JSON output parsed with no NaN or Infinity values.

[BartWaardenburg]

Landed directly on main in 216094f.