[Due for payment 2026-07-06] Improve line chart styling with marker borders, gradient fill, and brand green color

[MelvinBot]

Description

The line charts (particularly Spend over time) feel unpolished. This issue tracks the following visual improvements:

1. Marker borders — Add borders/strokes to the data point markers on the line chart
2. Linear gradient fill — Add a linear gradient to the area/spread beneath the line
3. Brand green color — Switch the chart color from blue to Expensify's primary brand green to reinforce brand identity and feel more polished

These changes have been confirmed as feasible using the combination of victory-native and skia.

Slack thread: https://expensify.slack.com/archives/C07HPDRELLD/p1778752346310759

Issue Owner

Current Issue Owner: @puneetlath

[shawnborton]

Perhaps while we're at it, we can also update our pie charts to be donuts:

[melvin-bot]

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[JmillsExpensify]

Are we planning on picking this initiative up this week?

[ZhenjaHorbach]

Are we planning on picking this initiative up this week?

Looks like we already have a draft PR
#90781

[mhawryluk]

yes, I will continue the work this week (hopefully today)

[mhawryluk]

the PR is ready for review