fix(projects): a section can be dropped into, and a failed load says so

Two things from the same report — an external member who could see a
project but could not get work into it.

**A section with no tasks could not be filled by dragging.** Only the task
rows accepted a drop, so the target was always another row; a section with
none had nothing to aim at, and a section you have just made is empty by
definition, which is exactly when you want to put something in it. The
board has taken a drop on the column since it was written. The list now
does the same: the section is the target, and a drop on it puts the task at
the end. Dropping on a row still means "here exactly" and stops there
rather than bubbling to the section behind it.

The unsectioned block at the top is a target too, which is how a task comes
back *out* of a section — previously only possible if some other loose task
happened to be there to aim at.

**And the page said "Projektet findes ikke, eller du har ikke adgang til
det" for three different things.** `project` started null with no loading
state, so that sentence was what you saw before the first request returned,
what you saw when it failed, and what you saw when you genuinely had no
access — and the effect never asked again, so the wrong one stayed until
somebody reloaded. That is very likely what was reported: the container was
restarted twice while people had the app open, and a request that lands
mid-restart leaves the page insisting the project does not exist.

Four states now. Loading renders nothing, as the shell does. 404 keeps the
old sentence, since the server deliberately answers 404 for both "no such
project" and "not yours" and the interface must not guess between them.
Anything else says the request failed and offers to try again.

What is *not* a bug: a viewer cannot create or move tasks. Checked at the
API — an editor invited by link gets 201 on create, 201 on quick add and
200 on a move into a section; a viewer gets 404 on all of them, which is
the rule working. If the person reporting this cannot add tasks, their
membership is viewer rather than editor.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/src/lib/components/TaskList.svelte

@@ -50,6 +50,10 @@
 
 	async function onDrop(event, target) {
 		event.preventDefault();
+		// The section around this list also takes drops, for the coarse "into here"
+		// gesture and for sections with no rows to aim at. A drop that landed on a
+		// row has already said exactly where it goes, so it stops here.
+		event.stopPropagation();
 		const id = draggingId;
 		const below = overBelow;
 		draggingId = null;