docs: prepare Product Hunt and Show HN launch

[jasonsuhari]

Problem

GridBash is ready for a public Product Hunt and Show HN launch, but its public launch surfaces disagree. GitHub has v0.2.0 cross-platform artifacts while npm latest remains v0.1.6, the website and launch kit still contain Windows-only/preview copy, and Product Hunt-specific assets and current platform guidance are incomplete.

Requested behavior

• Synchronize the website, repository launch copy, release links, and platform claims with the actually installable release.
• Add a current Product Hunt submission kit, asset checklist, launch-day checklist, and safe Show HN guidance that reflects HN's human-authorship rules.
• Create a square Product Hunt thumbnail and at least two gallery images using real GridBash product visuals.
• Document a safe manual npm bootstrap sequence that publishes the missing Windows native package before advancing the root package's latest tag.
• Keep unsigned macOS status and the npm registry blocker explicit until their existing issues are closed.

Acceptance checks

• No stale v0.1.6-only, Windows-only, or v0.2.0 preview link remains in the launch-facing site/kit unless clearly labeled as historical context.
• Product Hunt assets satisfy the documented dimensions and file-size limits.
• The npm procedure verifies identity, package availability, and dist-tags after each state-changing command.
• Show HN instructions do not provide generated text for direct posting and prohibit vote/comment solicitation.
• Launch-facing links and metadata pass focused validation.