Gate yaak-web to wasm32 and make the DB lock cover every worker

CI runs `cargo test --all`, which compiled yaak-web for the host and failed
in sqlite-wasm-rs's C shim. The crate is now `#![cfg(target_arch = "wasm32")]`
with its browser-only dependencies target-scoped, so it is empty natively.

Review caught a race in the worker fallback: a shared worker that starts
after the tab has given up on it and taken a dedicated worker would open the
database with no lock. The Web Lock now guards every worker, shared or not —
requested with a short timeout so a reloading tab's dying predecessor is
waited out, then reported. The tab also closes the port it abandoned. Verified
in production builds: 8/8 shared and 3/3 dedicated reloads render, and a
second tab in dedicated mode is told the database is in use.

packages/platform/src/web/connection.ts

@@ -137,6 +137,12 @@ export class WorkerConnection {
    * only then gives up on sharing and takes a dedicated worker.
    */
   private replaceWorker(why: string): void {
+    // Let go of the port that never answered. If a slow shared worker does
+    // come up later, it will find its port closed and — since it takes the
+    // same lock the replacement takes — cannot open the database beneath us.
+    if (this.port instanceof MessagePort) this.port.close();
+    else this.port.terminate();
+
     if (this.sharedAttempts < MAX_SHARED_ATTEMPTS) {
       console.warn(`Reconnecting to the database worker (${why})`);
       this.port = this.connectShared();