Move upstream README mirror out of src/ into content/

[kitsuyui]

Why

src/ is expected to hold this app's implementation modules, but the
upstream README mirror (src/readme.md, rendered on the homepage and
checked for sync by scripts/check-readme-sync.ts) lived in the same
directory. Mixing runtime code and mirrored documentation content in
one source root makes it harder for contributors to tell which src/
files are app modules versus copied data, and it forces the README
sync tooling to special-case a non-code file inside the app module
tree.

What changed

• Moved src/readme.md to content/readme.md.
• Updated every reference to the old path:
  • src/index.tsx (the ?raw import)
  • scripts/check-readme-sync.ts
  • scripts/check-readme-sync.test.ts
  • scripts/check-readme-html-elements.ts
  • README.md

No behavior changes: the README content, sync check, and HTML-element
check still operate on the same file, only its path changed.

Verification

• bun test scripts/check-readme-sync.test.ts — 14 pass, 0 fail.
• bun scripts/check-readme-html-elements.ts — passes against the
  relocated file.
• Verified by grep that no remaining reference to src/readme.md
  exists in the repository.
• Biome lint (bun run lint) and vite build could not be executed
  in this sandbox because the platform-specific biome/vite
  binaries failed to materialize during bun install here; the
  import path change is a mechanical rename verified via source
  inspection and the passing unit/script checks above.

[github-actions]

Code Metrics Report

	main (6219157)	#980 (69b5a27)	+/-
Coverage	91.2%	91.2%	0.0%
Code to Test Ratio	1:0.9	1:0.9	0.0
Test Execution Time	13s	17s	+4s

Details

  |                     | main (6219157) | #980 (69b5a27) | +/-  |
  |---------------------|----------------|----------------|------|
  | Coverage            |          91.2% |          91.2% | 0.0% |
  |   Files             |              3 |              3 |    0 |
  |   Lines             |            114 |            114 |    0 |
  |   Covered           |            104 |            104 |    0 |
  | Code to Test Ratio  |          1:0.9 |          1:0.9 |  0.0 |
  |   Code              |            306 |            306 |    0 |
  |   Test              |            288 |            288 |    0 |
- | Test Execution Time |            13s |            17s |  +4s |

Code coverage of files in pull request scope (90.9% → 90.9%)

Files	Coverage	+/-	Status
src/index.tsx	85.7%	0.0%	modified
src/readme-html-sanitizer.ts	91.2%	0.0%	modified

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Reported by octocov