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.

crates/yaak-web/Cargo.toml

@@ -16,15 +16,22 @@ wasm-opt = false # Matches yaak-templates; wasm-opt has caused errors in CI
 [lib]
 crate-type = ["cdylib", "rlib"]
 
+# The whole crate is `#![cfg(target_arch = "wasm32")]`: on a native target it
+# is empty, so a workspace-wide `cargo test` neither builds SQLite's wasm shim
+# for the host (which fails) nor links a browser-only runtime. Everything that
+# only exists for wasm is a target-scoped dependency for the same reason.
+
 [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 }
+yaak-models = { workspace = true }
+
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+console_error_panic_hook = "0.1"
+js-sys = "0.3"
+serde-wasm-bindgen = "0.6.5"
 sqlite-wasm-rs = "0.5"
 sqlite-wasm-vfs = "0.2"
 wasm-bindgen = "0.2.100"
 wasm-bindgen-futures = "0.4"
-yaak-models = { workspace = true }

crates/yaak-web/src/lib.rs

@@ -15,6 +15,11 @@
 //! its model store coherent, and blob storage. Sending, plugins, git, sync and
 //! everything else with a socket or a filesystem behind it lives elsewhere.
 
+// Nothing in here means anything off wasm32, and building it there would drag
+// SQLite's wasm C shim into a native compile. So on any other target the crate
+// is empty — a workspace-wide `cargo test` passes through it.
+#![cfg(target_arch = "wasm32")]
+
 use std::cell::RefCell;
 use std::sync::mpsc;