Add touch-first zoom controls to the Memory graph

[hamzamerzic]

Summary

• implement two-pointer pinch zoom anchored between the fingers on both Memory graph views
• add accessible zoom and reset controls with responsive gesture guidance
• keep the map-color legend compact on phones and reduce header crowding on small screens
• cover anchored scaling, pinch movement, and zoom bounds with focused tests

Why

The graph disabled native touch gestures but only tracked one pointer, so phone pinch gestures were swallowed instead of changing the graph scale. The fixed interaction owns all active pointers and keeps the graph point between the fingers stable while zooming and panning.

Testing

• node --test tests/test_graph_viewport.mjs (3 passed)
• pytest -q (106 passed)
• app validation (0 errors, 0 warnings)
• rendered at desktop and phone sizes; exercised zoom/reset controls, note opening, and the local graph flow