Windows: accept files dropped on the island

[JavaScriptUalist]

Summary

• Dragging a file from Explorer onto the island does nothing (Windows: dropping a file on the island does nothing ("no drop" cursor, no drag lines in the log) #126): WebView2 owns the innermost window, so Tauri's drop target never sees the file.
• The island now takes the drop in the page. The file contents go to a new ingest_bytes command and land in the inbox, with the same names and the same week-long sweep as a normal copy.
• While the island is hidden, the thin wake strip grows into a drop zone. Releasing a drag somewhere else closes the drop view, the buttons on the choose card stay clickable, and Explorer's drag picture stays in front of the island.

Test plan

• Drag a file from Explorer onto the open island: the cursor offers a copy, the log shows drag enter then drag drop, and the file appears in %LOCALAPPDATA%\Coucou\inbox.
• Drag a file to the top centre of the screen while the island is hidden: the island wakes and accepts the file.
• Drag a file over the island, then release it somewhere else: the drop view closes.
• Click +: the drop panel stays open.
• After a drop, Cancel and Ask a question both respond.
• A folder, and a file over 200 MB, are refused with a note.

cargo check --release and tsc --noEmit are clean on this branch. The drop path is the one confirmed on #114 for Windows 11 26H2 and WebView2 154; this brings it onto current main and keeps the shell drag picture above the island.

Fixes #126

Made with Cursor

[Louis-CFM]

Thank you for digging into #126! It was fixed in Coucou for Windows 0.2.0 (https://github.com/Louis-CFM/coucou/releases/tag/windows-v0.2.0) through #240. This approach wasn't taken because its drop-zone watch wakes every 50 ms and re-orders Explorer's drag window every 2 s for as long as the app runs, and the app has to stay at 0 % CPU when the island is hidden. Details in #307.

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