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/Cargo.toml

@@ -0,0 +1,30 @@
+[package]
+name = "yaak-web"
+version = "0.1.0"
+edition = "2024"
+publish = false
+
+# The desktop's model layer, compiled for a browser tab. See src/lib.rs.
+#
+# Building needs a clang with a WebAssembly backend: sqlite-wasm-rs compiles
+# sqlite3.c to wasm at build time, and Apple's clang cannot target it. See
+# build-wasm.cjs, which points cc at Homebrew LLVM when it is present.
+
+[package.metadata.wasm-pack.profile.release]
+wasm-opt = false # Matches yaak-templates; wasm-opt has caused errors in CI
+
+[lib]
+crate-type = ["cdylib", "rlib"]
+
+[dependencies]
+console_error_panic_hook = "0.1"
+js-sys = "0.3"
+log = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+serde-wasm-bindgen = "0.6.5"
+serde_json = { workspace = true }
+sqlite-wasm-rs = "0.5"
+sqlite-wasm-vfs = "0.2"
+wasm-bindgen = "0.2.100"
+wasm-bindgen-futures = "0.4"
+yaak-models = { workspace = true }