Add Classic Light mode and independent accent themes

[engelben]

Motivation

• Introduce a proper two-dimensional appearance system that separates base color mode (system | light | dark) from independent Pokémon accent palettes so users can choose brightness and accent independently.
• Prevent visible theme flash by applying the resolved appearance before React paints and migrate the legacy theme preference into the new accent storage safely.
• Provide a polished Classic Light token set while preserving the existing dark appearance and Pokémon visual effects.
• Make semantic color tokens available to components and Tailwind so shared UI surfaces can adapt to both light and dark modes.

Description

• Added an appearance hook API (useAppearance, exported also as useTheme) that manages colorMode, resolvedColorMode, accent, persistence keys pokecollector-color-mode and pokecollector-accent, legacy theme migration, live prefers-color-scheme tracking, and centralized DOM application. (frontend/src/hooks/useTheme.js)
• Added a small defensive pre-render initialization script to index.html that resolves mode+accent and applies them on the document element before React renders to avoid theme flash. (frontend/index.html)
• Introduced semantic CSS tokens for dark and Classic Light bases and accent variables, updated many shared component styles to consume the new tokens, and added per-accent definitions (including Electric foreground safety). (frontend/src/index.css)
• Updated Tailwind configuration to read semantic values from CSS variables and use token-based shadows and borders. (frontend/tailwind.config.js)
• Refactored the Settings UI to separate Appearance into accessible Color mode and Accent controls, added English and German translations, and wired the UI to the new appearance API. (frontend/src/pages/Settings.jsx, frontend/src/i18n/en.js, frontend/src/i18n/de.js)
• Updated toast styling to use semantic tokens and kept compatibility aliases (such as data-theme) to avoid breaking third-party/custom styles. (frontend/src/main.jsx)

Testing

• Ran git diff --check to validate whitespace and patch issues and it returned clean results.
• Performed a full frontend install and build with cd frontend && npm ci && npm run build and the production build completed successfully (Vite build completed).
• All tests passed (47 tests passed across the suite).