Chart had no way to read the value at a specific date — users had to
infer Saved amounts from line position relative to the y-axis labels
added in the previous commit.
- Transparent <rect> overlay covers the plot area + catches pointer.
- pointermove uses d3.bisector to snap to the nearest saved series
point, draws a dashed crosshair + a saved-line dot + a projection-line
dot (linearly interpolated between today and target).
- HTML tooltip lives inside the chart root (cleared on next _draw)
showing "date / Saved: $X / Projected: $Y". Clamps to viewport so it
doesn't overflow the card.
- pointerleave hides everything.
Pointer events unify mouse + touch — single handler covers both
desktop hover and mobile tap-and-drag. No keyboard nav yet; tracked as
follow-up (Stimulus controller is the right home but won't ship in
this round).
e03204bb96