feat(demo): add newsletter web archive example (#22)

[rajarshidattapy]

Summary

Adds a newsletter web archive example: the existing NewsletterDigest content rendered as a standalone web page with <Page>, from the same component tree as the email.

Closes #22.

Changes

• templates/NewsletterDigest.tsx — extracted newsletterTemplate(output), which swaps only the root wrapper (<Email> → tables for inboxes, <Page> → div/flexbox for the browser). Everything between the wrapper tags is shared; the footer links are the one intentional difference (Unsubscribe · View in browser vs Subscribe · Browse the archive).
• templates/NewsletterWebArchive.tsx — new: the web variant, registered in the template showcase with mode: "web" so it gets Preview / Code / HTML / Plain Text tabs.
• sections/NewsletterArchiveDemo.tsx — new "Send the email, publish the archive" section rendering both outputs side-by-side with the code that produces them.
• hooks/useRenderTemplate.ts — honour the entry's declared mode instead of always rendering as email (one line; was a latent bug for any non-email entry).

Test Plan

• pnpm build passes
• pnpm test passes (450 tests, 36 files)
• Tested in Storybook — N/A, demo app only, no library change
• Added/updated tests for new behavior — see Notes

Notes

Demo-only change; no library code touched. CI type-checks the demo via tsc -b in pnpm build.

Verified the section live in Chromium: both iframes render, no console errors. I also ran a throwaway parity check confirming email emits <table>, web emits none, both carry identical article content, and NewsletterWebArchive() matches newsletterTemplate("web") byte for byte — then deleted it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.50%. Comparing base (adaa68c) to head (aa08d07).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #51   +/-   ##
=======================================
  Coverage   95.50%   95.50%           
=======================================
  Files          31       31           
  Lines        1937     1937           
  Branches      382      382           
=======================================
  Hits         1850     1850           
  Misses         86       86           
  Partials        1        1           

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[ivoIturrieta]

Looks good! thanks for the contribution

[rajarshidattapy]

@ivoIturrieta thanks! Will be looking to contribute more :)