chore: upgrade react-email to ^6.1.4 (#1206)

* chore: upgrade react-email to ^6.1.4

Bump `react-email` from `^5.2.10` to `^6.1.4` in the web package.
react-email 6 bundles the preview server into the main package and
drops its Next.js dependency, so `@react-email/preview-server` and
the `@react-email/preview-server/next` resolution override are no
longer needed. Net `yarn.lock` impact is -302 MiB.

Also bumped the `esbuild` root resolution from `^0.27.3` to `^0.28.0`
since react-email 6 requires the matching host/binary, and bumped
the satellite ranges on `@react-email/components` (`^1.0.2` -> `^1.0.12`)
and `@react-email/render` (`^2.0.0` -> `^2.0.8`).

Smoke tested with `yarn workspace @sourcebot/web build` (Next.js
compile passes) and `yarn dev:emails` (preview server boots clean
at 6.1.4).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* chore: changelog entry for #1206

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Update CHANGELOG

Removed the mention of dropping the separate @react-email/preview-server devDependency in the changelog.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>