Tick labelling overhaul

[akenshaw]

This PR moves axis/grid math out of the UI layer into a shared, UI-agnostic engine in data::chart::ticks, now used by kline, comparison, heatmap, and indicator chart axes.

Time axis (X):

• Label spacing is denser, and the grid step adapts more smoothly while
  zooming (the step whose actual spacing lands closest to the target is
  picked, instead of the first one that roughly fits)
• Day/month/year boundaries are now always shown and highlighted, and are
  thinned to the pixel budget so long ranges stay readable
• The grid anchors to midnight and stays stable while panning,
  so the set of labels no longer shifts around as the view moves

Price axis (Y):

• Labels now land exactly on real price rows instead of between them, and
  are computed in exact units (no more f32 rounding drift)
• Label precision follows the market's minimum tick size