feat: add comments feature with nested replies and recursive rendering

Implement full comments system: domain entities (Comment, CommentLike),
value objects (CommentContent), use cases (CRUD, like toggle), SQLAlchemy
repository, API v1 endpoints, web UI with comment form and nested replies,
i18n translations (EN/RU/FR/DE), and E2E tests.

Fix nested reply (reply-to-reply) not displaying — the flat reply_comments
dict was only queried for top-level comment IDs, so deeply nested replies
were saved to DB (incrementing comment count) but never rendered. Switch
to a recursive Jinja2 macro that renders any nesting depth.

static/css/components.css

@@ -362,3 +362,179 @@ input[type="checkbox"] {
   margin-bottom: 1rem;
   opacity: 0.5;
 }
+
+/* Comment section */
+.comments-section {
+  margin-top: 2rem;
+  padding-top: 1.5rem;
+  border-top: 1px solid var(--color-border);
+}
+
+.comments-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: 1.25rem;
+}
+
+.comments-title {
+  font-size: 1.25rem;
+  font-weight: 600;
+  margin: 0;
+}
+
+.comments-count {
+  font-weight: 400;
+  color: var(--color-text-light-3);
+  font-size: 0.9375rem;
+}
+
+.comment-form-wrapper {
+  margin-bottom: 1.5rem;
+  padding: 1rem;
+  border: 1px solid var(--color-border);
+  border-radius: 6px;
+  background-color: var(--color-secondary-bg);
+}
+
+.comment-form .form-group {
+  margin-bottom: 0.75rem;
+}
+
+.form-textarea {
+  width: 100%;
+  padding: 0.625rem;
+  border: 1px solid var(--color-border);
+  border-radius: 4px;
+  background-color: var(--color-input-bg);
+  color: var(--color-text);
+  font-size: 0.875rem;
+  font-family: inherit;
+  line-height: 1.5;
+  resize: vertical;
+  min-height: 5rem;
+  box-sizing: border-box;
+}
+
+.form-textarea:focus {
+  outline: none;
+  border-color: var(--color-primary);
+  box-shadow: 0 0 0 3px var(--color-primary-alpha-30);
+}
+
+.form-help {
+  font-size: 0.8125rem;
+  color: var(--color-text-light-3);
+  margin-top: 0.375rem;
+  margin-bottom: 0;
+}
+
+.form-actions {
+  display: flex;
+  gap: 0.5rem;
+  align-items: center;
+}
+
+.comment-error {
+  margin-top: 0.5rem;
+  padding: 0.5rem 0.75rem;
+  background-color: var(--color-error-bg);
+  border: 1px solid var(--color-error-border);
+  border-radius: 4px;
+  color: var(--color-error-text);
+  font-size: 0.8125rem;
+}
+
+/* Individual comment */
+.comment {
+  display: flex;
+  gap: 0.75rem;
+  padding: 0.75rem 0;
+}
+
+.comment + .comment {
+  border-top: 1px solid var(--color-border);
+}
+
+.comment-avatar {
+  flex-shrink: 0;
+  padding-top: 0.125rem;
+}
+
+.comment-body {
+  flex: 1;
+  min-width: 0;
+}
+
+.comment-meta {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  margin-bottom: 0.25rem;
+}
+
+.comment-author {
+  font-weight: 600;
+  font-size: 0.875rem;
+  color: var(--color-text);
+}
+
+.comment-date {
+  font-size: 0.75rem;
+  color: var(--color-text-light-3);
+}
+
+.comment-content {
+  font-size: 0.875rem;
+  line-height: 1.5;
+  color: var(--color-text);
+  margin-bottom: 0.375rem;
+  word-wrap: break-word;
+}
+
+.comment-actions {
+  display: flex;
+  gap: 0.5rem;
+}
+
+.btn-comment-reply,
+.btn-comment-like {
+  font-size: 0.75rem;
+  padding: 0.25rem 0.5rem;
+  cursor: pointer;
+}
+
+.comment-replies {
+  margin-top: 0.5rem;
+  padding-left: 1rem;
+  border-left: 2px solid var(--color-border);
+}
+
+.comment-reply {
+  padding: 0.5rem 0;
+}
+
+.comment-reply + .comment-reply {
+  border-top: 1px solid var(--color-border);
+}
+
+.comments-empty {
+  text-align: center;
+  padding: 2rem 1rem;
+  color: var(--color-text-light-3);
+  font-size: 0.9375rem;
+}
+
+.btn-cancel-reply {
+  background: none;
+  border: none;
+  color: var(--color-primary);
+  cursor: pointer;
+  font-size: inherit;
+  padding: 0;
+  text-decoration: underline;
+}
+
+.btn-cancel-reply:hover {
+  color: var(--color-primary-hover);
+}