fix(votes): position filter no longer drags empty chairs into the view
Follow-up to !79 (merged) (merged earlier today): selecting Pour/Contre/etc. in the hemicycle legend showed the filtered choice PLUS every empty chair — as if the rule were 'filter pick + absents'.
Now: a position filter shows ONLY that position's votes; empty chairs join the view only when 'absent' is selected too. Party-filter behaviour unchanged (selected party's amber non-voters).
Simulated all five scenarios on law 705's scrutin: no filter / pour / absent / party / party+pour — each behaves as intended.