Fix blank custom Pages caused by incorrect template context

[next-devin]

Custom Pages returned HTTP 200 with empty document titles, breadcrumbs, headings, and body content because Spark referenced flatpage, while NEXT supplies the object as page. Replace the four references in templates/pages/page.html and record the fix in the unreleased changelog. Page content now renders with Account Only mode enabled as well.

Validation:

• All 67 existing Python tests pass.
• Committed CSS compatibility and CSS drift checks pass; CSS is unchanged.
• Verified the same template patch on three deployed custom Pages at 1440px desktop and 390px mobile: headings and full content render, with no horizontal overflow. Store-specific screenshots are retained locally and omitted from this public PR.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

The PR replaces the four flatpage references in templates/pages/page.html with the platform-provided page context variable and records the fix under a new ## Unreleased section in CHANGELOG.md. The change is minimal and surgical: only the wrong identifier is touched, surrounding markup and class names are unchanged, and grep confirms no remaining flatpage references in the repo. The changelog entry is consistent with prior entry style (single bullet, root cause + user impact, no extraneous framing).

Files Reviewed (2 files)

• templates/pages/page.html
• CHANGELOG.md

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