Fix: chart scenes unreadable on dark themes

[TROY665]

Fix: chart scenes unreadable on dark themes

Companion to #469 ("keep themed text legible on light playbooks"), which threads
theme colours into the text overlay components (CaptionOverlay, SectionTitle,
StatReveal, HeroTitle). This does the same for the chart components and
ComparisonCard, which #469 explicitly leaves untouched — same root cause
(hardcoded colour defaults that Explainer.tsx never overrides), opposite
direction (near-black defaults invisible on dark themes, rather than near-white
defaults invisible on light ones).

Both touch Explainer.tsx but in different JSX blocks, so whichever lands second
should rebase cleanly.

Problem

Explainer.tsx passes backgroundColor={bgColor} to the chart components but
never passes textColor. All four chart components — BarChart, LineChart,
PieChart and KPIGrid — default textColor to "#1F2937" (near-black).

On any dark theme the chart title, axis scale, category labels and value labels
are therefore drawn dark-on-dark and are invisible. The data renders correctly;
it simply cannot be seen. Bars, gridlines and series colours are unaffected,
which makes the failure look like "the labels aren't being passed" rather than a
colour bug.

ComparisonCard has the mirror-image version of the same problem: it does
receive textColor, but hardcodes cardBackgroundColor = "#F3F4F6". A dark
theme's light text then lands on a light card. Same symptom, opposite cause.

Reproduction

Render any explainer-data composition on the default dark theme with a
bar_chart or comparison scene:

{
  "type": "bar_chart",
  "title": "Neurons (millions)",
  "showValues": true,
  "chartData": [
    { "label": "Octopus", "value": 500 },
    { "label": "Dog", "value": 530 }
  ]
}

Before: bars and gridlines only — no title, no axis scale, no labels, no values.
After: all text visible.

Fix

• Forward textColor={textColor} to BarChart, LineChart, PieChart and KPIGrid.
• Pass cardBackgroundColor={cut.cardBackgroundColor || theme.surfaceColor} to
  ComparisonCard so the inner card follows the active theme.
• Add cardBackgroundColor?: string to the Cut interface, so a scene can still
  force a light card on a dark theme when a design calls for it.

Six lines, one file. No behaviour change on light themes: the components already
default to the same near-black, so forwarding an explicitly light-theme
textColor produces the identical result.

Tests

Adds two contract tests to tests/contracts/test_remotion_video_transition_contract.py,
following the existing source-assertion style in that file:

• test_chart_scenes_receive_the_theme_text_color — asserts each of the four
  chart components receives textColor={textColor}
• test_comparison_card_surface_follows_the_theme — asserts ComparisonCard gets
  a themed cardBackgroundColor

Both fail against the pre-fix Explainer.tsx and pass after it, verified by
stashing the change and re-running.

Verification

Rendered a 7-scene explainer (hero_title, text_card, stat_card, bar_chart,
comparison, callout) on the default dark theme, before and after. Before, the
bar chart showed bars and gridlines only and the comparison card's labels were
invisible. After, the chart shows its title, y-axis scale (0–530), category
labels and per-bar values, and the comparison card shows both labels on a themed
surface. The other five scenes are pixel-identical.

npx tsc --noEmit passes.