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.

crates/yaak-web/build-wasm.cjs

@@ -0,0 +1,79 @@
+const { execSync, spawnSync } = require("node:child_process");
+const fs = require("node:fs");
+const os = require("node:os");
+const path = require("node:path");
+
+// Same shape as crates/yaak-templates/build-wasm.cjs, plus one wrinkle: this
+// crate links SQLite, and sqlite-wasm-rs compiles sqlite3.c to wasm at build
+// time. That needs a C compiler with a WebAssembly backend, which Apple's
+// clang is not. So the build looks for one, and when it finds none it keeps
+// the committed pkg/ and says so — desktop developers never need this crate
+// rebuilt, and failing their `npm run bootstrap` over it would be wrong.
+
+if (process.env.SKIP_WASM_BUILD === "1") {
+  console.log("Skipping wasm-pack build (SKIP_WASM_BUILD=1)");
+  return;
+}
+
+/** A clang that can emit wasm32, or null. */
+function findWasmClang() {
+  const candidates = [
+    process.env.CC_wasm32_unknown_unknown,
+    "/opt/homebrew/opt/llvm/bin/clang", // Homebrew LLVM, Apple Silicon
+    "/usr/local/opt/llvm/bin/clang", // Homebrew LLVM, Intel
+    "clang", // Linux distros' clang usually has the backend built in
+  ].filter(Boolean);
+
+  for (const clang of candidates) {
+    const probe = spawnSync(clang, ["--print-targets"], { encoding: "utf8" });
+    if (probe.status === 0 && /\bwasm32\b/.test(probe.stdout)) return clang;
+  }
+  return null;
+}
+
+const clang = findWasmClang();
+if (clang == null) {
+  console.log(
+    [
+      "yaak-web: no C compiler with a WebAssembly backend found; keeping the committed pkg/.",
+      "  To rebuild: install LLVM (macOS: `brew install llvm`) or point CC_wasm32_unknown_unknown at one.",
+    ].join("\n"),
+  );
+  return;
+}
+
+// llvm-ar lives next to clang in every LLVM distribution
+const ar = path.join(path.dirname(clang), "llvm-ar");
+
+// Remap machine-specific paths that rustc embeds into the binary (panic
+// location strings), so builds are reproducible across machines
+const sysroot = execSync("rustc --print sysroot").toString().trim();
+const cargoHome = process.env.CARGO_HOME ?? path.join(os.homedir(), ".cargo");
+
+execSync("wasm-pack build --target bundler", {
+  stdio: "inherit",
+  cwd: __dirname,
+  env: {
+    ...process.env,
+    CC_wasm32_unknown_unknown: clang,
+    AR_wasm32_unknown_unknown: fs.existsSync(ar) ? ar : (process.env.AR_wasm32_unknown_unknown ?? ""),
+    RUSTFLAGS: `--remap-path-prefix=${cargoHome}=/cargo --remap-path-prefix=${sysroot}=/rustc`,
+  },
+});
+
+// Rewrite the generated entry to use Vite's ?init import style instead of
+// the ES Module Integration style that wasm-pack generates, which Vite/rolldown
+// does not support in production builds.
+const entry = path.join(__dirname, "pkg", "yaak_web.js");
+fs.writeFileSync(
+  entry,
+  [
+    'import init from "./yaak_web_bg.wasm?init";',
+    'export * from "./yaak_web_bg.js";',
+    'import * as bg from "./yaak_web_bg.js";',
+    'const instance = await init({ "./yaak_web_bg.js": bg });',
+    "bg.__wbg_set_wasm(instance.exports);",
+    "instance.exports.__wbindgen_start();",
+    "",
+  ].join("\n"),
+);