feat(web): show deployed commit SHA in landing footer

[cooper (czxtm)]

Summary

• Inject the commit SHA (short, 7 chars) into the web bundle at build time via Vite's define block. Reads process.env.GITHUB_SHA first (set automatically by GitHub Actions) and falls back to git rev-parse --short HEAD for local builds. If both fail, falls back to the literal string "unknown".
• Render the SHA in the landing footer's bottom row, next to the disclaimer, as a small monospace link to github.com/darkmatter/stackpanel/commit/<sha>. Lets you eyeball whether prod is currently serving the revision you expect.
• Added apps/web/src/types/build-info.d.ts declaring the __COMMIT_SHA__ global so TypeScript is happy.

Scope is intentionally just the landing footer for now — studio has its own chrome and can be extended later if useful.

Test plan

• Pull the branch, run bun run dev from apps/web, confirm the footer shows the local short SHA and clicking it opens the right GitHub commit URL.
• After deploy, hit the production landing page and confirm the SHA matches the main HEAD that triggered the deploy.

https://claude.ai/code/session_01WGbZGB93xMgBpBmoUDk91w

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Generated by Claude Code

[github-actions]

Preview deployed to pr-31 — https://pr-31.stackpanel.com

[cursor]

PR Summary

Low Risk
Low risk: build-time metadata injection and a small footer UI tweak; main risk is misreporting SHA in non-git/CI environments (falls back to unknown).

Overview
Adds a build-time __COMMIT_SHA__ constant (sourced from GITHUB_SHA or git rev-parse --short HEAD, else "unknown") via Vite’s define config.

Updates the landing footer to show the deployed SHA as a monospace link to the corresponding GitHub commit, and adds a build-info.d.ts declaration so TypeScript recognizes the injected global.

^{Reviewed by Cursor Bugbot for commit 7dd303c. Configure here.}