Migrate remaining BEM components to CSS Modules (final batch)

Migrate the last 4 BEM components to CSS Modules, completing the
migration and removing bem-react-helper from the project entirely.

Components migrated:
- subscribe-by-email (1 BEM CSS file -> 1 module)
- comment-form + markdown-toolbar (20 BEM CSS files -> 2 modules)
- comment (19 BEM CSS files -> expanded existing module)
- root (10 BEM CSS files -> expanded existing module)

Consolidates ~50 BEM CSS files into 4 new + 2 expanded CSS Module files.
Removes bem-react-helper dependency — all components now use clsx for
conditional class composition.

Dead CSS cleanup during migration:
- Orphaned comment-actions selectors in comment theme CSS (already migrated)
- Dead BEM modifiers: comment_disabled, comment_pinned, comment_guest
- Dead element: comment__user-id (CSS existed but never used in TSX)
- Dead button type classes: comment-form__button_type_preview/_send
- Dead mix values: auth-email-login-form__back-button, comment-form__email-dropdown

Key implementation details:
- comment_highlighting stays global via :global() (imperatively added by classList)
- Bare .dark/.light theme class preserved on root wrapper (8+ modules depend on it)
- raw-content.css kept as global utility CSS (syntax highlighting)

Visual regression verification on built artefacts:
- remark.css: 43,779 -> 36,106 bytes (-17.5%)
- last-comments.css: 18,792 -> 13,955 bytes (-25.7%)
- remark.js: 256,709 -> 253,637 bytes (-1.2%)
- last-comments.js: 121,726 -> 120,795 bytes (-0.8%)
- Total: 441,006 -> 424,493 bytes (-3.7%)
- Screenshot comparison: pixel-identical across light/dark themes

Author: paskal