fix(sections): a section can hold more than one task

Reported as "sections can only have one task", and that is exactly what it looked
like.

TaskList read the dragged task from its own `draggingId`, which is only set when
the drag *started* in that list. A task dragged in from the unsectioned rows above,
or from another section, arrived as null and the handler returned early — after it
had already called `stopPropagation`, so the section around it never got the drop
either.

So a section could only be dropped into while it was empty, because an empty one
has no row in the way to swallow the drop. Put one task in and there was no route
to a second. The id comes out of `dataTransfer` at drop time now, and `dragover`
decides with `carries()` rather than local state — which is the only thing a target
can read while a drag is in the air.

The stuck frame was the same cause: the section's own cleanup never ran, so the
highlight stayed lit after the drop. It is cleared on `dragend` now, which fires
once per drag on the source and bubbles to the window — the one place that is true
however a drag ends, including one abandoned with Escape, which produces no drop at
all and left the frame standing too.

The smoke test drops a second task onto the row already in the section, which is
the gesture that did nothing, and runs in WebKit as well.

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

web/e2e/smoke.spec.js

@@ -1203,6 +1203,75 @@ test('færdige opgaver kan vises og skjules igen', async ({ page }) => {
 	expect(trouble).toEqual([]);
 });
 
+/**
+ * A section holds more than one task, and stops being highlighted afterwards.
+ *
+ * Both reported from use, and both the same root cause. TaskList read the dragged
+ * task from its *own* `draggingId`, which is only set when the drag started in
+ * that list — so a task dragged in from the unsectioned rows arrived as null and
+ * the handler returned early. It had already called `stopPropagation`, so the
+ * section around it never got the drop either.
+ *
+ * The result was a section you could only drop into while it was empty, because
+ * an empty one has no rows to aim at. Put one task in, and there was no way to add
+ * a second: it looked exactly like a section that holds one task.
+ *
+ * The same `stopPropagation` left the section's frame lit, since the handler that
+ * clears it never ran.
+ */
+test('en sektion kan rumme flere opgaver, og rammen forsvinder bagefter @forms', async ({
+	page
+}) => {
+	const trouble = watchForTrouble(page);
+	await page.goto('/');
+
+	const sidebar = page.getByRole('navigation', { name: 'Hovedmenu' });
+	await sidebar.getByRole('button', { name: 'Nyt projekt' }).click();
+	await sidebar.getByLabel('Projektnavn').fill('Badeværelset');
+	await sidebar.getByLabel('Projektnavn').press('Enter');
+	await expect(page.getByRole('heading', { name: 'Badeværelset' })).toBeVisible();
+
+	await page.getByRole('button', { name: '+ Tilføj sektion' }).click();
+	await page.getByLabel('Ny sektion').fill('Håndværker');
+	await page.getByLabel('Ny sektion').press('Enter');
+	await expect(page.getByRole('heading', { name: 'Håndværker' })).toBeVisible();
+
+	for (const what of ['knager op', 'fuge om vinduet']) {
+		await page.getByLabel('Ny opgave').fill(what);
+		await page.getByLabel('Ny opgave').press('Enter');
+		await expect(page.getByText(what, { exact: true })).toBeVisible();
+	}
+
+	const section = page.locator('section').filter({
+		has: page.getByRole('heading', { name: 'Håndværker' })
+	});
+	const row = (what) => page.locator('.row').filter({ hasText: what });
+
+	// The first goes in while the section is empty — the only drop that used to
+	// work, because there was no row in the way to swallow it.
+	await row('knager op').dragTo(section);
+	await expect(section.getByText('knager op', { exact: true })).toBeVisible();
+
+	// And the second, aimed at the row that is now in there. This is the one that
+	// did nothing at all.
+	await row('fuge om vinduet').dragTo(section.getByText('knager op', { exact: true }));
+	await expect(section.getByText('fuge om vinduet', { exact: true })).toBeVisible();
+	await expect(section.locator('.row')).toHaveCount(2);
+
+	// The frame is a drop target being offered, not a state. Once the drag is over
+	// it has nothing left to say.
+	await expect(section).not.toHaveClass(/\bover\b/);
+
+	// It survives a reload, so this is the server's answer and not the page's.
+	await page.reload();
+	const after = page.locator('section').filter({
+		has: page.getByRole('heading', { name: 'Håndværker' })
+	});
+	await expect(after.locator('.row')).toHaveCount(2);
+
+	expect(trouble).toEqual([]);
+});
+
 /**
  * The sidebar never scrolls sideways.
  *