feat: redesign dashboard messages UI with better sticker/image display and AI filtering

MessageCard:
- Add sticker image preview (renders actual sticker from URL, not just text name)
- Show attachment thumbnails inline (up to 4 with overflow counter)
- Remove dead columns (ai_policy_version, ai_evidence) from UI
- Add severity color coding (critical=red, high=orange, medium=yellow, low=blue)
- Add relative time display ('2h ago' instead of full datetime)
- Better hover effects and visual hierarchy

MessagesPanel:
- Add stats bar showing total/clean/warn/flagged/error/pending/deleted/edited counts
- Add AI status filter buttons (all, clean, warn, flagged, error, pending)
- Improve search UX with inline search icon and clear button
- Show filtered count in tab labels

ImageGrid:
- Include sticker images (was only attachments + embeds before)
- Add kind badge overlay (sticker/attachment/embed)
- Show user avatar next to image caption
- Better sticker rendering (object-contain with padding)

Header:
- Add shield icon per tab
- Improve tab titles and add descriptive subtitles

API client:
- Remove dead fields from MessageRecord (ai_moderation_raw, ai_policy_version, ai_evidence)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -26,14 +26,11 @@ export interface MessageRecord {
   ai_status?: AIStatus | null;
   ai_moderation_flags?: string | null;
   ai_moderation_score?: number | null;
-  ai_moderation_raw?: string | null;
   ai_analysis?: string | null;
   ai_categories?: string | null;
   ai_severity?: AISeverity | null;
   ai_confidence?: number | null;
   ai_recommended_action?: AIRecommendedAction | null;
-  ai_policy_version?: string | null;
-  ai_evidence?: string | null;
   ai_analyzed_at?: number | null;
   ai_error?: string | null;
 }