Mobile UI: Phase 2 - Unify chart tooltips and add tap-toggle for touch

[JeffreyBLewis]

Part of the mobile UI plan (see Phase 1 issue for overview).

Goal

Chart point/legend details currently only show on mouseover via d3.tip, duplicated across five files. On touch devices there's no hover, so this detail is invisible on mobile.

Affected: static/js/voteCharts.js (~line 528), static/js/party.js (~lines 428, 494), static/js/committee.js (~line 347), static/js/personIdeology.js (~line 253), static/js/district.js (~line 375).

Scope

• Extract the repeated d3.tip mouseover pattern into one shared helper.
• Gate on matchMedia('(pointer: coarse)'): mouse/pointer-fine keeps hover behavior; touch gets tap-to-toggle (tap opens, tap elsewhere or a close affordance dismisses).
• Ensure toggled tooltips are keyboard/focus accessible too (consistent with the existing WCAG pass).

Test plan

• On desktop (mouse), hover behavior is unchanged
• On a touch device/emulator, tapping a point/legend entry shows its tooltip; tapping elsewhere dismisses it
• No regression in the scatter, party glance, committee, person ideology, or district pages

🤖 Generated with Claude Code

https://claude.ai/code/session_013KJMYfgTNsxrmjHZpZKAHT

[JeffreyBLewis]

Overview/Phase 1: #448