#896 [BUG] Color accuracy: approximate OKLab transform + grayscale light theme charts

Bug Report

Platform

All platforms (Linux, macOS, Windows)

Current Behavior

In apps/desktop/src/theme.rs:150-181, the OKLab to sRGB conversion uses approximate hardcoded coefficients that produce visibly incorrect colors:

let red = linear_srgb_to_srgb(4.076_741_7 * l - 3.307_711_6 * m + 0.230_969_94 * s);
let green = linear_srgb_to_srgb(-1.268_438 * l + 2.609_757_4 * m - 0.341_319_4 * s);
let blue = linear_srgb_to_srgb(-0.004_196_086_3 * l - 0.703_418_6 * m + 1.707_614_7 * s);

These are simplified approximations of the OKLab inverse transform. For a video editor, color accuracy is critical - these inaccuracies will compound across the rendering pipeline.

Additionally, light theme chart colors are all grayscale (chroma=0):

chart_1: oklch(0.87, 0.0, 0.0),  // All chart_1 through chart_5 have chroma=0
chart_2: oklch(0.556, 0.0, 0.0),
chart_3: oklch(0.439, 0.0, 0.0),
chart_4: oklch(0.371, 0.0, 0.0),
chart_5: oklch(0.269, 0.0, 0.0),

This makes charts unusable in light mode - all bars/lines render as identical gray.

Expected Behavior

Use the exact OKLab inverse transform matrix from the specification (Björn Ottosson's paper)
Light theme chart colors should have distinct hues (chroma > 0)

Steps To Reproduce

Run desktop app in light mode
Observe chart components - all colors are identical gray
Compare rendered colors against reference OKLab implementation

Recurrence Probability

Always - deterministic color math error

Additional Context

For a video editor, color fidelity is a core requirement. Consider using a vetted color space library like palette or oklab crate instead of hand-rolled approximations. The current implementation deviates from the OKLab spec.