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/components/markdown/Markdown.tsx

@@ -3,7 +3,6 @@ import ReactMarkdown, { type Components } from 'react-markdown'
 import rehypeRaw from 'rehype-raw'
 import remarkGfm from 'remark-gfm'
 import { cn } from '@/lib/utils'
-import { MentionHoverCard } from '@/components/common/mention-hover-card'
 import { CodeBlock, InlineCode } from './CodeBlock'
 import { preprocessLinks } from './linkify'
 
@@ -61,15 +60,10 @@ function createComponents(
     a: ({ href, children }) => {
       // Mention links: mention://member/id or mention://agent/id
       if (href?.startsWith('mention://')) {
-        const parts = href.replace('mention://', '').split('/')
-        const mentionType = parts[0] ?? 'member'
-        const mentionId = parts[1] ?? ''
         return (
-          <MentionHoverCard type={mentionType} id={mentionId}>
-            <span className="text-primary font-semibold mx-0.5">
-              {children}
-            </span>
-          </MentionHoverCard>
+          <span className="text-primary font-semibold mx-0.5">
+            {children}
+          </span>
         )
       }