feat(core): theme service refactor (#DS-3003)

[NikGurev]

Summary

Refactors ThemeService (DS-3003): moves it to signals, adds a built-in auto mode that follows the OS color scheme, and persists the selected mode to localStorage out of the box. ThemeService is kept as a deprecated alias so existing consumers don't break, and the new KbqThemeService is fully DI-configurable via kbqThemeProvider().

List of notable changes

• added KbqThemeService — signal-based (mode, resolvedMode, currentTheme, themes) replacement for ThemeService, with setAuto()/toggle(), internal matchMedia handling for auto mode, and localStorage persistence via a new swappable KBQ_THEME_STORE/KbqThemeLocalStorageStore (same pattern as KBQ_ACCORDION_STATE_STORE)
• added kbqThemeProvider(config) / KBQ_THEME_CONFIG for DI-based setup (themes, mode, storageKey, autoLight, autoDark) instead of imperative setThemes()/setTheme() calls
• added autoLight/autoDark config so auto mode resolves correctly against fully custom theme sets, not just the built-in light/dark names
• kept ThemeService (deprecated alias of KbqThemeService), KbqTheme.selected (deprecated, still synced), and current/setTheme()/getTheme() (deprecated, still functional) for backward compatibility — no ng update schematic needed for the rename
• removed the docs app's hand-rolled matchMedia listener and localStorage wiring in navbar.component.ts/navbar-property.ts — now just calls setAuto()/setMode(); configured with kbqThemeProvider({ storageKey: 'docs_theme' }) in apps/docs/src/app/config.ts to preserve existing users' saved preference under the old key
• updated all other ThemeService consumers (theme-toggle.ts, welcome.component.ts, tokens-overview.ts, docsearch.directive.ts, 7 docs-examples) to the signal API
• added theme.service.spec.ts — unit tests for auto resolution, mode selection, custom themes, persistence, SSR-safety, and the deprecated shims
• updated docs/guides/migration.en.md / migration.ru.md with a new "13. Theme service review (20.3.0)" section, and approved the core.api.md public API snapshot

What should reviewers focus on?

• Whether the deprecated back-compat surface (ThemeService, current, selected, setTheme/getTheme) is worth keeping vs. a cleaner break
• autoLight/autoDark as the mechanism for custom themes to opt into auto mode — reasonable default, or should it be required when custom themes are registered?

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