BL-15958 Rename ebook theme to edge-to-edge, expand to all page sizes

[hatton]

[Claude Fable 5 following a prompt from Hatton]

Part 1 of 3 of the updated BL-15958 work, split out of #7713.

Tracker: BL-15958

What this does

• Renames the zero-margin-ebook appearance theme to edge-to-edge. AppearanceSettings normalizes the old name as an alias, so existing books keep working.
• Expands the theme from Ebook/Device layouts to every page size, in support of full-bleed print.
• Fixes page-number placement for the theme. New CSS variables --pageNumber-side-left-inset / --pageNumber-side-right-inset place side page numbers, with full-bleed offsets; the generated override formula also keeps master's --pageNumber-forced-side-extra-margin term (BL-16695).
• Updates the theme label in all localization files, template appearance.json files, and the visual regression test theme list.

Relationship to the other parts

Independent of the other two PRs; based on master. The siblings are the full-bleed page-size PR and the edit-mode crop-marks PR.

Verification

• vite build passes.
• The AppearanceSettingsTests, AppearanceMigratorTests, and BookTests fixtures pass (243 tests).

🤖 Generated with Claude Code

Devin review

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This change is 

[greptile-apps]

Greptile Summary

This PR renames the zero-margin ebook appearance theme to Edge to Edge, normalizes the legacy theme ID, and extends its margin and page-number behavior to all page sizes.

• Adds legacy-to-canonical theme-name normalization in AppearanceSettings.
• Renames and broadens the appearance-theme CSS, including full-bleed text and page-number insets.
• Updates templates, migration data, tests, and localization catalogs for the new theme name.

Important Files Changed

Filename	Overview
src/BloomExe/Book/AppearanceSettings.cs	Normalizes the legacy theme alias on loading, assignment, and CSS generation, and updates explicit side-page-number formulas.
src/content/appearanceThemes/appearance-theme-edge-to-edge.css	Renames the theme, applies zero margins across page sizes, and introduces full-bleed text and page-number inset rules.
src/content/bookLayout/basePage.less	Adds an inset frame for top-level translation groups so full-bleed text can remain within the safe area.
src/content/bookLayout/pageNumbers.less	Refactors the full-bleed bottom offset into a separately composed CSS variable.
DistFiles/localization/en/BloomMediumPriority.xlf	Renames an established trans-unit ID, which breaks the repository’s localization identity and retirement workflow.
DistFiles/localization/es/BloomMediumPriority.xlf	Directly rewrites a Crowdin-owned locale while retaining an approved translation for the old theme meaning under the new key.
src/BloomTests/Book/AppearanceSettingsTests.cs	Updates expectations for canonical theme naming and the new page-number inset formulas.
src/BloomVisualRegressionTests/index.spec.ts	Replaces the old theme ID with the canonical ID in the visual-regression theme matrix.

_{Reviews (1): Last reviewed commit: "BL-15958 Rename ebook theme to edge-to-e..." | Re-trigger Greptile}

[greptile-apps]

Theme rename loses translations

When the updated catalog is synchronized with Crowdin or used in a non-English locale, replacing the established non-translate="no" trans-unit ID and directly rewriting translated catalogs loses the stable translation identity and associates old “Zero Margin Ebook” targets with the new “Edge to Edge” key. Preserve the old unit as obsolete and add the new unit only to the English catalog.

Context Used: AGENTS.md (source)

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