Fix #359: web drops land in the document they were dropped on

The web reads dropped files asynchronously, and the place inbox only kept
the drop point, so a file dropped on document A was placed into whatever
document was active when its bytes arrived. A drop on a canvas now carries
place::DropAt (the document's uid, the point, embed), and drop_files makes
that document active again before placing; when it has closed meanwhile,
nothing is placed and the status bar says so. The native drop path is
synchronous and goes through the same code, so it is unchanged in effect.

Test: a drop pending while another document opens lands in the first one
at the drop point; one whose document closes is not placed anywhere.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/development.md

@@ -20,7 +20,7 @@ How the web shell (`apps/vectorcraft-web/src/web.rs`) differs from desktop:
 
 - **Open** sets `Services::open_async`, which shows `rfd::AsyncFileDialog`. The bytes arrive in `Services::inbox`, which the app drains every frame.
 - **Save / Save As / Export** go through `Services::download`: a Blob, an object URL and a temporary `<a download>`, all created from Rust. There is no save dialog, so the suggested name becomes the download name.
-- **Drag-and-drop:** `WebShell` takes the frame's `dropped_files` before the app sees them, reads each with `DroppedFile::bytes_async` and pushes the bytes into the inbox. (The app's synchronous drop path is compiled out on wasm32.)
+- **Drag-and-drop:** `WebShell` takes the frame's `dropped_files` before the app sees them, reads each with `DroppedFile::bytes_async` and pushes the bytes into the inbox (files dropped on a canvas go to `Services::place_inbox` with their `place::DropAt`: the document they were dropped on, by uid, and the point). A file that arrives after another document became active still lands in its own document, which becomes active again; if that document closed meanwhile, the file is not placed and the status bar says so. (The app's synchronous drop path is compiled out on wasm32.)
 - **No control server:** browsers can't listen on TCP. To automate the web build, drive headless Chrome with `--remote-debugging-port`.
 - Quick smoke test: `"/Applications/Google Chrome.app/Contents/MacOS/Google Chrome" --headless=new --enable-unsafe-webgpu --screenshot=web.png --window-size=1440,900 --virtual-time-budget=15000 http://127.0.0.1:8766/` (headless Chrome on macOS gets a real WebGPU adapter).