fix(comments): replace optimistic updates with loading state

- Remove temp-xxx optimistic inserts from submitComment/submitReply
- Wait for API response, then insert real comment into timeline
- Add Loader2 spinner to comment/reply submit buttons during loading
- Remove hover card from Markdown.tsx (will be handled via NodeView later)

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

apps/web/features/issues/components/comment-input.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useRef, useState } from "react";
-import { ArrowUp } from "lucide-react";
+import { ArrowUp, Loader2 } from "lucide-react";
 import { Button } from "@/components/ui/button";
 import { RichTextEditor, type RichTextEditorRef } from "@/components/common/rich-text-editor";
 
@@ -44,7 +44,7 @@ function CommentInput({ onSubmit }: CommentInputProps) {
           disabled={isEmpty || submitting}
           onClick={handleSubmit}
         >
-          <ArrowUp className="h-4 w-4" />
+          {submitting ? <Loader2 className="h-4 w-4 animate-spin" /> : <ArrowUp className="h-4 w-4" />}
         </Button>
       </div>
     </div>