feat(core): support global and contextual downIsGood configuration (#3297)

[FrancescoCastaldi]

Closes #3297

Description

This PR introduces global, page-level, and wrapper-context downIsGood (down_is_good) default configuration for <Delta>, <BigValue>, <Table>, and related comparison components.

In cost and finance dashboards, an increase is often negative (costs went up) and a decrease is positive (costs went down). Previously, authors had to add down_is_good={true} to each individual component.

Changes

1. Svelte Context & <DeltaDefaults> tag:
   • Created DeltaDefaultsContext (delta-defaults.context.svelte.ts)
   • Added <DeltaDefaults down_is_good={true}> / <DeltaDefaults downIsGood={true}> wrapper component
2. Page Frontmatter & Project Layout:
   • Added down_is_good and downIsGood to projectRootPageFrontmatterSchema and projectLayoutSchema
   • Updated pageSettingsSchema and CLI page settings resolution to pass layout & frontmatter defaults
3. Theme Defaults:
   • Added defaults and delta blocks to themeConfigSchema and themeOverridesSchema
4. Resolution Cascading in DeltaDisplay:
   • Priority hierarchy:
     1. Component prop override (comparison.down_is_good / downIsGood)
     2. Nearest <DeltaDefaults> context
     3. Page frontmatter (down_is_good / downIsGood)
     4. Project layout / theme defaults
     5. Fallback: false
5. Tests:
   • Added unit test suite delta-defaults.test.ts for schema validation and default inheritance
   • Added test cases in files.server.test.ts

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[FrancescoCastaldi]

Hi @archiewood @hughess! 👋 Whenever you have a moment, could you please take a look at this PR and approve the CI workflow? Thank you!