Release 2.4.1: five answers to the intraday chart feedback

[CtrlAltDevelop]

Closes #3.

Five changes, one for each point raised in the issue. Each is its own commit.

Fit a short series to the width — ChartStyle.fitContent

pointWidth is a fixed distance, so a handful of intraday bars bunched up
against the left edge and left the rest of the chart empty. Fitting widens the
spacing to fill the plot, and widens the candle and volume bars with it so they
keep their proportions. It only ever widens, so a series long enough to fill the
plot is laid out on pointWidth as before and the flag can stay on while
history pages in.

A fitted series is exactly as wide as the plot, so getMinTranslateX holds the
scroll at zero rather than leaving the half point the last candle's centre
normally needs.

Keep a price the locked axis cannot reach inside the candle area — bug fix

A locked axis holds the range it was given, so a tick beyond it had nowhere of
its own to be drawn: the line was painted over the volume and indicator panes,
or off the canvas where it was invisible.

A horizontal line at such a price is now left out of the plot, and its label is
pinned to the edge the price went past and marked with an arrow so the level can
still be found. The current-price line, the signal lines and the trading tags
are clamped to the same edge — they were clamped to the window's extremes,
which under a lock are not the axis's.

Keep the newest candle on a locked axis — lockedScaleFollowsPrice

Opt-in, so an existing lockPriceScale: true chart behaves exactly as before.
The locked range grows just enough to cover the newest candle and never shrinks
back or refits to the window, so the axis still sits still while the chart is
scrolled. Only the newest candle counts, and only while it is in view: growing
the axis to swallow the history a scroll moves over would undo the lock a little
at a time.

Write the chart's prices — priceFormatter

fixedLength only said how many decimals a price is written to, so a currency
symbol, a thousands separator or a tick size had nowhere to go. The new callback
is the counterpart to dateFormatter and covers every price the chart itself
says: the axis labels, the crosshair's price label, the current-price tag, the
high, low and signal tags, and the OHLC legend. An axis that reads out a move
rather than a price writes that move itself and does not ask.

A second price axis — secondaryPriceAxisScale

The chart had one price axis, so reading a move as a percentage meant giving up
the prices. A second axis reads the same candles another way, in a gutter on the
side verticalTextAlignment left free and sized by
ChartStyle.secondaryPriceAxisWidth. It marks its own round values rather than
labelling the price axis's, the grid stays ruled by the price axis, and the
crosshair and price tags keep following priceAxisScale. The two gutters are
clamped against each other, so between them they can never take more than half
the chart's width.

Also in here

• Background fill fix. Each band was filled from the canvas edge for the
  plot's width, so a gutter on the right showed whatever was under the widget
  and a gutter on the left shifted the fill. Every band now spans the canvas.
• Example app. A toggle for each change, and the intraday demo uses
  fitContent instead of working the spacing out by hand.
• Goldens for the fitted series (before and after), the second axis, the
  currency prices and the edge-marked level.
• One existing perf test priced its drawings below everything on screen, which
  no longer resolves an anchor now that an unreachable line is skipped before
  the lookup. Repriced around the newest candle; its subject is unchanged.

Release

Version 2.4.1, with the changelog entry and the README's install line updated.

Testing

898 tests pass, flutter analyze and dart format clean, and the example
builds for both entry points. 22 new tests, including three that fail without
the out-of-range fix.