Update litmus-web to provider-scoped theme rendering (litmus-y6dc)

Migrate litmus-web from the old Theme struct to ThemeDefinition + ProviderColors.

Global Provider Selector

Provider becomes app-level state, not a per-page control. Switching provider changes simulated colors, screenshots, contrast validation, and which themes are visible.

UI: Segmented control (buttons, not dropdown) at the top of sidebar, above all filters. E.g. [kitty] [wezterm]. Styled as a prominent first-class mode selector.

State persistence:

• URL parameter takes precedence (e.g. /themes?provider=kitty)
• Falls back to localStorage
• Falls back to first available provider
• Shareable/bookmarkable links

Filtering: Selecting a provider filters the theme list to only themes with ProviderColors for that provider. Themes without colors for the selected provider are hidden everywhere (list, detail, compare).

Navigation edge case: If user is on a detail page and switches to a provider that doesn’t have that theme, redirect to theme list.

Data Loading

• Update load_embedded_themes() to return Vec + per-provider color map
• Theme only listed if it has ProviderColors for the active provider

Rendering

• Simulated scenes render using ProviderColors for the active provider
• Screenshots use active provider
• Contrast validation scoped to active provider’s colors
• Theme list cards show previews using active provider’s colors
• Compare page uses active provider for all columns

State Management

• New global signal: ActiveProvider(String) (like existing CvdSimulation, VariantFilter)
• Remove per-page provider dropdowns (detail page, compare page)
• All components read from global ActiveProvider

Depends on: new model types, converted themes

Plan

Phase 1: Update themes.rs data loading

• Replace single THEME_DATA array with two arrays: DEFINITION_DATA and PROVIDER_COLORS_DATA
• Parse using parse_theme_definition() and parse_provider_colors()
• Add load_embedded_theme_data() → (Vec, HashMap<Key, ProviderColors>)
• Add available_providers() → sorted list of provider slugs
• Add themes_for_provider(provider) → Vec

Phase 2: Add ActiveProvider signal to state.rs

• New signal: ActiveProvider(String)
• URL param: ?provider=kitty
• localStorage fallback
• Default: first available provider

Phase 3: Update components

• Shell: theme chrome uses active provider
• Sidebar: add provider selector (segmented control)
• ThemeList: filter by active provider
• ThemeDetail: use active provider for rendering + screenshots
• Compare: use active provider
• SceneAcross: use active provider

Phase 4: Navigation edge cases

• Provider switch on detail page → redirect if theme unavailable

Todo

• Update themes.rs with provider-based loading
• Add ActiveProvider signal
• Add provider selector UI
• Update all pages to filter by provider
• Handle navigation edge cases (graceful fallback — detail page shows “not found” with link back)
• Compiles for wasm32, zero warnings

Summary of Changes

Phase 1: Rewrote themes.rs with two embedded arrays (DEFINITION_DATA + PROVIDER_COLORS_DATA), OnceLock caching, and new API: themes_for_provider(), available_providers(). Converted 6 remaining old-format themes (cyberdream, melange, light-owl, oxocarbon-light) to ThemeDefinition + provider colors.

Phase 2: Added ActiveProvider signal to state.rs with default-to-first-available provider.

Phase 3: Added provider segmented control in sidebar. Updated all 4 pages (ThemeList, ThemeDetail, Compare, SceneAcross) and Shell to read from ActiveProvider signal.

Phase 4: ThemeDetail gracefully shows “not found” with back link when theme unavailable for selected provider. Shortlist items persist across provider switches (raw slug shown if name unavailable).

All 169 tests pass, zero compiler warnings, wasm32 compilation verified.