feat: Skip analytics store for long-term tracking (optional)

[smithrashell]

Summary

Add a skips store to IndexedDB for long-term skip analytics and pattern tracking.

Note: This is an optional enhancement that builds on #244. It requires a database migration.

Context

Issue #244 implements skip functionality without persistence. This issue adds optional long-term tracking to enable analytics.

Would Enable

• Track skip frequency per problem
• Analyze which problems are commonly skipped
• Skip patterns over time
• Insights into problem difficulty calibration
• Dashboard analytics for skip behavior

Schema

// New store: 'skips'
{
  id: auto-increment,
  leetcode_id: number,
  session_id: string,
  reason: 'too_difficult' | 'dont_understand' | 'not_relevant' | 'other',
  other_text: string | null,
  timestamp: ISO string,
  had_relationships: boolean // for "free skip" tracking
}

Requirements

• Database version increment (47 → 48)
• Migration handling in src/shared/db/index.js
• New skipService.js for analytics queries
• Dashboard integration (optional)

Files to Create/Modify

• src/shared/db/index.js - Add skips store, increment version
• src/shared/services/session/skipService.js (new) - Analytics queries
• src/background/handlers/problemHandlers.js - Persist skips

Potential Analytics Queries

// Get most skipped problems
async function getMostSkippedProblems(limit = 10) { ... }

// Get skip reasons distribution
async function getSkipReasonDistribution() { ... }

// Get skips by time period
async function getSkipsByDateRange(startDate, endDate) { ... }

Priority

Low - This is a nice-to-have enhancement. The core skip functionality (#244) works without it.

Labels

enhancement, database, future, low-priority

Depends On

• feat: Enhanced skip functionality with reason tracking and smart behavior #244 (Enhanced skip functionality)