fix(comments): fix padding, separate CommentInput from ReplyInput

- Remove Card double padding (Card py-4 + inner px-4 was too much, override with !py-0)
- Use lighter border-border/50 for reply separators inside Card
- Create CommentInput component for bottom "Leave a comment" — no avatar, full
  width editor, submit button in footer row below editor
- ReplyInput stays for in-card "Leave a reply" — has avatar, compact inline layout
- Two different components for two different use cases

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/features/issues/components/index.ts

@@ -3,3 +3,6 @@ export { PriorityIcon } from "./priority-icon";
 export { StatusPicker, PriorityPicker, AssigneePicker, DueDatePicker } from "./pickers";
 export { IssueDetail } from "./issue-detail";
 export { IssuesPage } from "./issues-page";
+export { CommentCard } from "./comment-card";
+export { CommentInput } from "./comment-input";
+export { ReplyInput } from "./reply-input";