implemented custom theme system

[adeboladee]

Closes #379

PR Summary: Subscription Custom Theme System

Key Features

• Theme Configuration: Utilizes ThemeConfig with brand colors (primary, secondary, accent + semantic colors), FontConfig (family, weights, sizes), and LogoConfig (URI, darkURI, dimensions).
• Real-time Preview: State machine managed in the store following the flow:
  startPreview → updatePreview → applyPreview / discardPreview.
• CSS Variable Generation: Automatically generates --st-{name} variables with auto -rgb variants, --st-ext-{name} for extended colors, font/logo variables, and full :root stylesheet generation.
• WCAG Accessibility: Automatic contrast auditing on every custom theme creation using getAccessibilityRating(), providing fix suggestions for non-compliant color pairs.
• Light/Dark Variant Pairs: The ThemeVariantPair system links light and dark themes together under a shared ThemeSharedConfig featuring fonts and logos.
• Theme Inheritance: inheritTheme() allows the creation of child themes from a parent with partial overrides.
• Export/Import: Introduces a JSON-based ThemeExportData format (v1.0.0) for sharing themes across merchants.
• API Persistence: Backend CRUD endpoints with complete merchant isolation and single-active-theme enforcement controlled via a PostgreSQL trigger.
• Auto-Semantic Colors: Automatically generates missing semantic colors (success, warning, error, info) from the primary brand color with mode-appropriate saturation.

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New Files Created

File	Purpose
src/theme/customThemeBuilder.ts	Utility for hex/RGB conversion, color blending, semantic palette generation, full theme construction from config, light/dark variant pair creation, and theme inheritance.
src/theme/cssVariables.ts	CSS variable generation from Theme objects with camelCase → kebab-case conversion, hex → RGB variable extraction, font/logo variables, and full stylesheet generation.
src/theme/accessibility.ts	WCAG 2.1 AA/AAA contrast checking (relativeLuminance, contrastRatio, meetsWcagAA/AAA), getAccessibilityRating() for a full theme audit, and suggestContrastFix() for auto-correction.
src/theme/themeService.ts	API service layer handles CRUD operations for themes with AsyncStorage persistence and saveThemeVariantPair().

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Modified Files

File	Changes
src/theme/types.ts	Added ExtendedThemeColors, FontConfig, LogoConfig, AccessibilityInfo, AccessibilityIssue, ThemeVariantPair, ThemeSharedConfig, ThemeConfig, ThemeExportData, ThemeInheritance, and ThemePreviewState.
src/theme/themes.ts	createBrandTheme now generates extendedColors, sets isCustom, manages createdAt/updatedAt, and exports highContrastTheme.
src/theme/themeStore.ts	Complete rewrite to support the preview state machine, updateCustomTheme(), addThemeVariantPair(), removeThemeVariantPair(), exportTheme(), importTheme(), syncToApi(), syncFromApi(), WCAG accessibility rating audits, getThemeById(), and getVariantPair().
src/theme/index.ts	Re-exports all new types and utilities.
src/store/index.ts	Added useThemeStore export.
backend/subscription/router/index.ts	Added createThemeRouter export.
backend/server/createApiServer.ts	Mounted theme router at /api/v1/merchant.
backend/subscription/router/themeRouter.ts	Express router mounting theme endpoints under /api/v1/merchant.
backend/migrations/004_theme_storage.sql	SQL migration creating merchant_themes and theme_variant_pairs tables with JSONB config, alongside triggers for single-active-theme enforcement.

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Test Results

83 tests passing across 6 test suites:

• themes.test.ts (6 tests): Verifies highContrastTheme, extendedColors, isCustom, and timestamps.
• customThemeBuilder.test.ts (21 tests): Covers color utilities, theme construction, variant pairs, and inheritance.
• cssVariables.test.ts (12 tests): Covers variable flattening, camelCase → kebab-case conversion, prefix handling, and stylesheet generation.
• accessibility.test.ts (20 tests): Covers luminance, contrast ratios, WCAG AA/AAA checks, theme ratings, and fix suggestions.
• themeStore.test.ts (16 tests): Expanded from 9 → 16 tests covering previews, variants, export/import, and inheritance.
• themeService.test.ts (7 tests): Covers CRUD, export/import, and sync capabilities.

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