docs(readme): add Modem sponsor block

[benvinegar]

Summary

• add a centered sponsor block below the README badges
• use a <picture> element so the combined Modem logo swaps between light and dark PNGs
• link the sponsor block to modem.dev with repo-specific UTM parameters

Notes

• copied logo assets into assets/ so the README preview works directly in the PR

This PR description was generated by Pi using GPT-5

[greptile-apps]

Greptile Summary

This PR adds a centered sponsor block to the README, featuring a <picture> element that swaps between light and dark Modem logos based on the user's color scheme preference, with UTM-tracked links to modem.dev. The two PNG assets are committed to assets/ for in-repo rendering.

Confidence Score: 5/5

Safe to merge — docs-only change with no logic, security surface, or breaking impact.

All changes are documentation and static binary assets. The HTML is valid, the picture/source ordering is correct, the fallback img uses an appropriate alt attribute, and UTM parameters are well-formed. No P0 or P1 findings.

No files require special attention.

Important Files Changed

Filename	Overview
README.md	Adds centered sponsor block with light/dark mode logo and UTM-tracked links using standard HTML picture element.
assets/modem-sponsor-dark.png	New binary asset: dark-mode sponsor logo for Modem.
assets/modem-sponsor-light.png	New binary asset: light-mode sponsor logo for Modem.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Browser renders README] --> B{prefers-color-scheme?}
    B -->|dark| C[modem-sponsor-dark.png]
    B -->|light| D[modem-sponsor-light.png]
    B -->|no preference / unsupported| E[fallback img: modem-sponsor-light.png]
    C --> F[Link → modem.dev with UTM params]
    D --> F
    E --> F

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