fix(ui): Clear button on column filter leaves (All) checkbox marked and removes table header on zero-row result (followup #482)

[cristim]

Followup to #482 — QA verification failed. Two sub-bugs in frontend/src/recommendations.ts:

1. Clear-categorical handler at lines 1830-1845 sets cb.checked = false directly on data checkboxes but never calls updateAllTriState(), so the (All) checkbox stays in its prior state. Compare to commitAll() at 1786-1796 which does call it.
2. When the filter is {kind:'set', values:[]}, applyColumnFilters returns 0 rows. renderRecommendationsList at 3556-3562 replaces the entire <table> with a <p class='empty'> element, so the <thead> is gone with the body — table header disappears.

Fix direction

(1) In the Clear-categorical branch (recommendations.ts:1841-1843) call commitAll(false) instead of duplicating the body, or at minimum invoke updateAllTriState() after flipping the checkboxes.
(2) On zero-row, render an empty <tbody> under the existing <thead> (so the header survives) plus an inline 'No rows match these filters' hint. Add visual-state test coverage at recommendations.test.ts:2208 (existing test only checks the state-setter call, not visual (All) survival).

Surfaced by QA spreadsheet row 4.17. PR #491 (closes #482) merged but verification failed.

[cristim]

Verified shipped: PR #712 (commit e7103d8, merged 2026-05-25) is in feat/multicloud-web-frontend. Fixes both sub-bugs in recommendations.ts: (All) tri-state checkbox updated after Clear, and table header preserved on zero-row result via empty tbody. Tests extended in recommendations.test.ts.