refactor: unify Astro frontend, modular GAS backend, and deployment workflows

[HeiTang]

Summary

This PR consolidates the recent ShortYou architecture work into main.

It replaces the legacy frontend with the Astro single-page flow, restructures the GAS backend into modular runtime components, and aligns deployment/configuration workflows across local and production environments.

Key changes

• migrate the frontend to Astro and unify the product flow around /, /#alias, and /#t=<token>
• remove the legacy invite flow and old static frontend assets
• modularize the GAS backend into config, controller, repository, services, types, utils, and request entrypoints
• rebuild the GAS runtime context per request so updated Script Properties are not reused from warm instances
• centralize public site URL handling and env-aware frontend/GAS workflows
• add GitHub Actions for Pages deployment and production health checks
• refresh English and Traditional Chinese READMEs to match the current architecture and operations model

Notes

• includes the cleanup commit that removes the outdated backend specification document
• branch source: dev
• target branch: main

[Copilot]

Pull request overview

This PR centralizes frontend site URL configuration, updates production URLs, adds production health checks, and refactors GAS runtime construction so backend config is reloaded per request.

Changes:

• Replaces hard-coded frontend canonical/CNAME/robots URLs with env-derived site URL handling.
• Adds production health-check scripts/workflows and updates deployment workflow inputs.
• Rebuilds GAS backend runtime context per request and makes PUBLIC_SITE_URL required for capability links.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/layouts/BaseLayout.astro	Derives canonical URL from Astro site config.
scripts/site-url.mjs	Adds shared site URL normalization and artifact generation.
scripts/run-frontend.mjs	Requires site URL env and writes frontend artifacts after build/preview.
scripts/check-production-health.mjs	Adds live frontend/GAS production validation script.
README.md	Updates public quick-start domain.
README.zh-TW.md	Updates public quick-start domain in Traditional Chinese.
public/robots.txt	Removes static robots file in favor of generated output.
public/CNAME	Removes static CNAME in favor of generated output.
package.json	Adds production health-check npm script.
gas/src/entrypoints.ts	Rebuilds GAS runtime context on each entrypoint/admin call.
gas/src/config.ts	Removes default public site URL and errors when missing for capability links.
docs/backend-spec.zh-TW.md	Removes outdated backend specification document.
astro.config.mjs	Uses env-derived site URL for Astro config.
.github/workflows/production-healthcheck.yml	Adds scheduled/manual production health-check workflow.
.github/workflows/pages.yml	Supplies site URL to frontend build and adds post-deploy health check.
.github/workflows/gas-cicd.yml	Updates production deploy env validation and deploy/sync command.
.env.prod.example	Updates production public site URL example.

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[Copilot]

Pull request overview

Copilot reviewed 17 out of 17 changed files in this pull request and generated 8 comments.

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