Wire up file open for the web build:
- `io::pick_and_load_web`: browser file picker → read bytes → parse via
`load_bytes` → build caches, returning the same payload as the native
loader so it feeds the shared `Message::FileOpened` handler.
- `Message::OpenFile` cfg-dispatches: native picks a path then loads on a
worker thread; web picks + parses in one async step. The web branch also
sets `opening` (the handler drops results when no open is in progress).
- Disable the cosmic-text shaping / fallback paths on wasm: the web has no
system fonts, so cosmic-text panics with "no default font found". LFF
stroke fonts (embedded) still render; LFF-missing glyphs are skipped.
Native build + 75 tests unchanged. Hatches still don't draw on the web.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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