Sort a data-table column once per heading click

[thunderballfists]

Clicking a data-table column heading delivers HeaderSelected twice, so the ascending/descending toggle always ends on descending and a table can never be sorted ascending by clicking.

Measured on current main (f8e472c) with real pointer clicks on the second heading of a small table:

click 1 -> HeaderSelected [1, 1]  rows ['a', 'c', 'b']  N ↓
click 2 -> HeaderSelected [1, 1]  rows ['a', 'c', 'b']  N ↓
click 3 -> HeaderSelected [1, 1]  rows ['a', 'c', 'b']  N ↓

With this change:

click 1 -> HeaderSelected [1]  rows ['b', 'c', 'a']  N ↑
click 2 -> HeaderSelected [1]  rows ['a', 'c', 'b']  N ↓
click 3 -> HeaderSelected [1]  rows ['b', 'c', 'a']  N ↑

Cause. Textual calls the _on_* handler of every class in the MRO. SeededDataTable._on_click and _on_mouse_move also called super(), which runs DataTable's own handler a second time.

Fix. Neither handler calls super() any more. _on_click only has to swallow the click that ends a resize press, and does that with event.prevent_default(), which is what keeps DataTable's handler from running for it. The resize-hover update in _on_mouse_move (from #87) is kept.

Tests (both use real Pilot clicks and both fail on main without the change):

• one click posts exactly one HeaderSelected, and repeated clicks toggle ascending, descending, ascending;
• the click that ends a resize press does not sort, and the next ordinary click does.

This was found by an application's test suite, which sorts tables by heading and fails against main; it passes against this branch.

Validation:

• full headless suite, Python 3.13 on macOS: 391 passed
• pyflakes textui: clean
• visual tests in a python:3.12-slim container with Textual 8.2.8: 6 passed (no snapshot changes)
• GitHub Actions on this PR covers Python 3.11, 3.12, 3.14, lint and visual

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