fix(webbluetooth): read characteristic values via the DataView's buffer

[cfanboy]

Fixes #941 (maintainer green-lit the PR in the issue thread).

Problem

In the WebBluetoothDeviceCommand::Read handler, readValue() resolves to a DataView, but the bytes were extracted with Uint8Array::new(&data_view). In JS, new Uint8Array(dataView) treats the DataView as a plain array-like (length === undefined), producing an empty array. The subsequent copy_to then trips js-sys's length assertion and panics, killing the whole wasm instance:

panicked at js-sys-0.3.104/src/lib.rs:14143:1:
assertion `left == right` failed
  left: 0
 right: 15
Uncaught RuntimeError: unreachable

Net effect: any protocol that performs a hardware read during init dies silently right after chooser pairing, and the embedded server is dead from that point on. Native btleplug paths are unaffected, which makes this confusing to diagnose from the outside.

Fix

Build the Uint8Array over the DataView's underlying buffer(), honoring its byte offset/length — the Subscribe notification handler already uses the buffer-based pattern. One expression changed, plus a comment explaining why Uint8Array::new must not be used here.

Verification

• cargo check -p buttplug_server_hwmgr_webbluetooth --target wasm32-unknown-unknown passes.
• Verified against a physical Sensee Capsule (CCPA10S2, protocol sensee-v2, which reads model data from Tx during init) on Chrome/macOS: with this patch in our wasm build, the device identifies and runs (vibrate + constrict) through the browser embedded server; without it, the panic above reproduces 100% after pairing.

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