fix(playground): raise demo preview limit to 50 MB

[CoderLambert]

Scope

• Increase the Playground-only preview policy from 2/3/5 MiB to 20/35/50 MiB (warning / confirmation / block).
• Apply the same 50 MiB maximum to remote URL downloads, avoiding a 100 MiB fetch before the preview gate blocks the file.
• Display the per-file limit near the local file selector.
• Document GitHub Pages manual acceptance checks, including the 50 MiB boundary.

Behavior

• Local files at or below 50 MiB can be previewed (confirmation required from 35 MiB).
• Local files above 50 MiB remain selectable but are blocked by LargeFileGate with its existing download fallback.
• Remote URL files above 50 MiB are rejected during the download stage.
• Shared @lamberl-lee/file-preview default policy is unchanged.

Validation

• Existing CI executes lint, typecheck, tests, and production build.
• Manual GitHub Pages acceptance checklist updated under docs/github-pages-release-checklist.md.

Resolves the reported Playground demo 5 MiB preview limitation.