chore: swap ko-fi button for markdown GitHub-style variant

[erwan-joly]

Replaces the legacy ko-fi HTML button with the official ko-fi.com/img/githubbutton_sm.svg markdown image so all NosCore repos render the same GitHub-style sponsorship row.

Summary by CodeRabbit

• Documentation
  • Updated the donation link presentation in project documentation.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 57b7ea4c-3c24-4033-9a37-e3e6d444f731

📥 Commits

Reviewing files that changed from the base of the PR and between 4101793 and 0d9b2d1.

📒 Files selected for processing (1)

• README.md

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Walkthrough

The Ko-fi donation section in README.md has been updated to replace an HTML anchor element containing a raster image with a Markdown-rendered Ko-fi "githubbutton_sm" image. The donation link URL remains unchanged at https://ko-fi.com/A3562BQV.

Changes

Cohort / File(s)	Summary
Documentation README.md	Replaced HTML <a> element with raster image for Ko-fi donation with Markdown-rendered Ko-fi button image. Destination URL unchanged.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: replacing an HTML Ko-fi button with a Markdown-rendered variant in the README.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/kofi-button-update

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