BrowserDb errors have no error code #673

BrowserDb (the browser feature, published as @minigraf/browser) rejects with the error's top-level message only, without the structured code that every other API carries:

await db.execute('(nosuchcommand [])')
// rejects with "Unknown command: nosuchcommand"
// the native API, the WASI REPL and the other bindings report "[PRS-…] Unknown command: nosuchcommand"

Cause: src/browser/mod.rs converts errors with JsValue::from_str(&e.to_string()) (12 places on main, 15 on v3), which formats the anyhow::Error without its code. Only the IndexedDB flush path goes through MinigrafError::from(e). Since #277 (v2.0.0), every error is supposed to carry its code (docs/ERROR_REFERENCE.md).

Fix: route every conversion through one helper, MinigrafError::from(e).to_string(), and add a wasm_bindgen_test that checks for the [XXX-NNN] prefix. Fix it on v3 first, then backport to main for v2.0.9.

Found by the minigraf-wasm consumer test (binding consumer tests plan, docs/superpowers/plans/2026-10-20-binding-consumer-tests.md).

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