README.md: Add responsive logo for light and dark themes

[masenf]

Updated logo display to support light and dark themes in a more standardized way.

https://stackoverflow.com/a/70200610

[greptile-apps]

Greptile Summary

This PR upgrades the README logo to support both light and dark color schemes by wrapping the existing <img> tag inside an HTML5 <picture> element with two <source> entries keyed on prefers-color-scheme.\n\nKey changes:\n- Dark mode renders reflex_light.svg (white-filled paths) — confirmed correct by inspecting the SVG fill colors.\n- Light mode renders reflex_dark.svg (#110F1F-filled paths) — confirmed correct.\n- Older browsers / clients that don't support <picture> fall back to the original reflex.svg.\n- All three referenced SVG files are present in docs/images/.\n- The width=\"300px\" attribute remains on the inner <img> tag, which is the correct HTML5 placement for <picture> — it applies to whichever source is ultimately rendered.

Confidence Score: 5/5

Safe to merge — documentation-only change with correct HTML5 picture element usage and all referenced assets verified to exist.

Single-file README change; the picture/source/img pattern is exactly the standard GitHub-supported approach for theme-aware images. Both SVG variants exist in the repo, fill colors match the intended usage, and the fallback img ensures compatibility with clients that don't support picture. No logic, runtime code, or dependencies are touched.

No files require special attention.

Important Files Changed

Filename	Overview
README.md	Replaces static img tag with a picture element that serves reflex_light.svg for dark mode and reflex_dark.svg for light mode, with reflex.svg as a fallback; both variant SVGs exist in the repo and have the correct fill colors.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Browser loads README] --> B{Supports picture element?}
    B -- No --> C[Render fallback img\nreflex.svg]
    B -- Yes --> D{prefers-color-scheme?}
    D -- dark --> E[Load reflex_light.svg\nwhite logo on dark bg]
    D -- light --> F[Load reflex_dark.svg\ndark logo on light bg]
    D -- not set --> G[No matching source\nFallback to reflex.svg]

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_{Reviews (1): Last reviewed commit: "README.md: Add responsive logo for light..." | Re-trigger Greptile}

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks

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_{Comparing masenf/readme-logo-light-dark (52f7467) with main (1e78f2c)}