fix(bear): preserve Unicode tags

[markoub]

Summary

• preserve Unicode letters and marks when Bear simple tags are normalized
• extract CJK nested tags such as #标签/子标签 into frontmatter instead of collapsing them to _
• move Bear tag normalization into a small helper with focused tests

Fixes #515

Validation

• pnpm install --frozen-lockfile
• pnpm exec tsx --test tests/bear/tags.test.ts
• pnpm run test
• pnpm run build
• pnpm exec eslint src/formats/bear-bear2bk.ts src/formats/bear-tags.ts tests/bear/tags.test.ts
• git diff --check