feat: add gamification to the crawl and graph exploration experience

[ChefControl]

Context

The project vision says the visualization "should be fun for the user, possibly gamified."

Currently there are no scores, achievements, milestones, or competitive elements.

Proposed Solution

• Real-time discovery counter that ticks up as URLs are found
• Milestone notifications ("100 URLs discovered!", "New domain found!")
• Crawl completion celebration (confetti animation or similar)
• Stats comparison between crawls
• Enhanced graph interactivity: node search, domain filtering, animated node discovery

Files to Modify

• frontend/src/pages/CrawlDetail.tsx — milestone notifications, discovery counter
• frontend/src/components/GraphView.tsx — animated node discovery, search/filter
• frontend/src/components/StatsView.tsx — comparison features

Acceptance Criteria

• Real-time discovery counter visible during active crawls
• At least one milestone/celebration animation
• Graph shows new nodes appearing with animation during active crawls

[ChefControl]

A workspace is ready for this issue.

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