dnd: fix drag-and-drop onto panel applets

[dahankzter]

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Blocked on Smithay/smithay#2146 — this cannot land until that merges and the smithay rev here is bumped.

Dropping a file onto a panel applet has never worked. Applets get enter and motion fine, then the drag ends in leave and wl_data_device.drop never arrives, which is why this looks like it should already work from reading the code.

Two separate causes, established from a WAYLAND_DEBUG capture on the panel:

1. action_choice returned preferred verbatim whenever no seat had a local dnd_source — every drag that didn't start inside the panel. A client may leave preferred empty and libcosmic does, so the action negotiated to "none" and cosmic-comp refused the drop. Now uses smithay's default_action_chooser.

2. Nothing relayed the applet's wl_data_offer.accept up to cosmic-comp. The host only sends drop once we've accepted on its offer. Source had no hook to observe that, hence the smithay PR.

The first two commits are independent and fine on their own. Only the third needs the new hook.

Tested on COSMIC 1.7.0, with a local [patch] pointing at a smithay fork (not part of this branch). Dropping onto the applet icon and onto a popup row opened mid-drag both deliver the path. On the wire: drop() from cosmic-comp, forwarded to the applet, receive("text/uri-list", fd) + finish() coming back.

Commits carry Co-Authored-By: Claude Opus 5 — this was written with Claude Code, with the diagnosis driven by wire captures rather than guesswork.

[jacobgkau]

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[dahankzter]

Sorry about that @jacobgkau it's back now