fix: add startup sync and dark chat text fix

[codedogQBY]

Summary

• Fix Android dark-mode AI answer text by forcing mobile markdown text nodes to inherit the active theme color.
• Add a separate sync-on-startup preference for WebDAV/S3 sync, with desktop and mobile settings toggles.
• Preserve the new preference across config saves and cover it in sync store tests.

Fixes #281
Fixes #278

Verification

• pnpm --filter @readany/core test -- src/stores/sync-store.test.ts
• pnpm --filter @readany/app-expo exec tsc --noEmit
• pnpm --filter app build
• pnpm exec biome check packages/core/src/sync/sync-backend.ts packages/core/src/stores/sync-store.ts packages/core/src/hooks/use-auto-sync.ts packages/core/src/stores/sync-store.test.ts packages/core/src/i18n/locales/zh/settings.json packages/core/src/i18n/locales/zh-TW/settings.json packages/core/src/i18n/locales/en/settings.json packages/core/src/i18n/locales/es/settings.json packages/core/src/i18n/locales/fr/settings.json packages/core/src/i18n/locales/ja/settings.json packages/core/src/i18n/locales/ko/settings.json packages/app/src/components/settings/SyncSettings.tsx packages/app-expo/src/screens/settings/SyncSettingsScreen.tsx packages/app-expo/src/components/chat/MarkdownRenderer.tsx

Note: the targeted Biome check passes with existing noExplicitAny warnings in MarkdownRenderer only. A full pnpm lint still reports unrelated pre-existing issues such as the bundled pdf.worker.mjs size limit and other files outside this change.