Run the desktop's model layer in the browser

The browser host now stores data through yaak-models compiled to wasm — the
same queries, migrations, cascade rules, duplicate naming and first-run
bootstrap the desktop and CLI use — instead of a TypeScript port of them.

- crates/yaak-web: the wasm crate. boot() registers an IndexedDB-backed VFS
  and calls init_standalone; rpc(cmd, payload, label) answers the models_*
  commands via ClientDb and returns the model_writes it caused; blob get/put
  through blob_manager. Built like yaak-templates (pkg/ committed); the
  build script keeps pkg/ and says so when no wasm-capable clang is present,
  so a desktop bootstrap never depends on one.
- models_ops moves from crates/yaak into yaak-models so the wasm crate can
  use it without the send engine.
- The database lives in a SharedWorker (packages/platform/src/web/worker.ts):
  one process holds the data and pushes writes to every tab, as on the
  desktop. Where SharedWorker is missing or its script cannot be fetched, a
  dedicated worker guarded by a Web Lock takes over and a second tab is told
  so. The worker imports the wasm lazily so a tab's connect is answered
  instantly; the tab reconnects if it isn't.
- packages/platform/src/web loses models.ts, schema.ts, db.ts and the
  BroadcastChannel; commands.ts forwards model commands to the worker and
  keeps the fixed answers and refusals.

Verified in Chrome, dev and production builds: bootstrap, CRUD across every
model type, serde defaults, engine-format ids, Rust copy naming, folder
cascade with per-descendant events, single-event workspace delete, reload
persistence, two tabs coherent, Send declined with a toast, 8/8 reloads
rendering in ~100 ms.

apps/yaak-client/commands/switchWorkspace.tsx

@@ -1,3 +1,4 @@
+import { platform } from "@yaakapp-internal/platform";
 import { createFastMutation } from "../hooks/useFastMutation";
 import { getRecentCookieJars } from "../hooks/useRecentCookieJars";
 import { getRecentEnvironments } from "../hooks/useRecentEnvironments";
@@ -24,7 +25,9 @@ export const switchWorkspace = createFastMutation<
       request_id: requestId,
     };
 
-    if (inNewWindow) {
+    // A host without windows opens the workspace here instead. Refusing would
+    // strand the user on the workspace they were trying to leave.
+    if (inNewWindow && platform.capabilities.multiWindow) {
       const location = router.buildLocation({
         to: "/workspaces/$workspaceId",
         params: { workspaceId },