Hover preview for wikilinks in chat messages

[ryuhmanov-m]

Parent issue: #34

Context

Chat messages can contain wikilinks ([[Note Name]]). Currently they render as plain text or simple links. Users want a hover preview showing the note's content.

Scope

1. Detect [[...]] patterns in rendered message content
2. On hover, show a popup/tooltip with:
   • Note title
   • First 3-5 lines of content (or first paragraph)
   • "Open in Knowledge" link
3. Fetch note content via existing kb_read tool or Tauri command
4. Add a small delay before showing (200-300ms) to avoid flickering
5. Dismiss on mouse leave

Design

• Use a floating tooltip/popover (existing UI patterns in the codebase)
• Keep it read-only — no editing in the preview
• Match existing dark/light theme

References

• Message rendering: src/components/chat/MessageBubble.tsx
• Markdown renderer used in messages
• Knowledge API: Tauri commands for reading vault files