fix(browser): preserve nullable decoded text

[ubugeeei]

Summary

• return text: null from browser/WASM decode results when core decoded text is absent
• update browser typings for nullable decoded text
• keep Vue decode and camera composables displaying text only when present

Closes #4

Validation

• pnpm install --frozen-lockfile
• rustfmt --check crates/fastqr_wasm/src/lib.rs
• cargo check -p fastqr-wasm --target wasm32-unknown-unknown
• pnpm --filter fastqr task:check
• pnpm --filter @fastqr/vue task:check

Note: direct tsc -p npm/fastqr_vue/tsconfig.json --noEmit still reports pre-existing strict type errors tracked separately in #18.

[ubugeeei]

Landed via #27 after local vp run ci and final GitHub Actions CI passed. Closing this issue-scoped PR to keep the queue and PR list clean.