🐛 Fixed theme editor launch gate for allowlisted themes on limited plans

[betschki]

• the server's POST /themes/upload/ blocks every save by a limited customer using a . sentinel that's never in the allowlist, so editing any theme on a plan with a customThemes limit enabled is rejected regardless of which theme is active
• the launch gate previously checked the active theme name against the allowlist, so customers whose current theme happened to be allowlisted could enter the editor only to hit a generic "something went wrong" toast on save
• the gate at all three editor entry points (Change theme menu, installed themes list, /theme/edit/:name deep link) now uses the same . sentinel as the upload endpoint, so the LimitModal fires at launch with the host's customThemes error message instead

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Walkthrough

This PR unifies the theme editor access control by consolidating how three components check editor limits. Previously, the logic conditionally passed either the '.' sentinel or the active theme name based on whether the theme was default or legacy. The change makes all three components (ChangeTheme, DesignAndThemeModal, AdvancedThemeSettings) consistently use the '.' sentinel when calling checkThemeLimitError, removing the conditional branching. Supporting imports are updated by removing isDefaultOrLegacyTheme and introducing isDefaultTheme where needed. A new acceptance test validates that editor access is blocked even when a theme is present in the custom themes allowlist.

Possibly related PRs

• TryGhost/Ghost#27656: Both PRs modify theme editor launch gating by changing how checkThemeLimitError is computed with the '.' sentinel and extend acceptance coverage for editor blocking when custom themes are limited.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: fixing the theme editor launch gate to properly block allowlisted themes on limited plans by using the '.' sentinel.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The pull request description clearly explains the bug fix: the launch gate for the theme editor now uses a '.' sentinel to align with server-side behavior, preventing users from entering the editor only to fail on save.

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✨ Finishing Touches

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