feat: add sort-by options (messages, hours, cost) to leaderboard

[yanis7774]

Summary

• Adds sort-by buttons (Prompts / Hours / Cost) to the global leaderboard, displayed next to the existing time-period tabs (Today / Week / All Time)
• Active sort metric is visually highlighted (purple underline) in each leaderboard row
• Sort buttons use purple accent to visually distinguish them from blue period tabs

Changes

• src/frontend/app.js — added _lbSortBy state, switchLbSort() function, updated renderGlobalBoard() sorting logic to use dynamic sort key, added sort buttons to leaderboard HTML, added .lb-sort-active class to active metric in stats
• src/frontend/styles.css — added styles for .lb-tabs-row, .lb-sort-tabs, .lb-sort-label, .lb-sort, .lb-sort-active

How it works

The sort selector works in combination with time-period tabs:

• Period tabs (Today/Week/All Time) control which time window to display
• Sort buttons (Prompts/Hours/Cost) control which metric to rank by

Closes #123

Test plan

• Open Leaderboard view
• Verify "Sort by: Prompts | Hours | Cost" buttons appear to the right of period tabs
• Click each sort button — leaderboard re-sorts by selected metric
• Verify active sort metric is highlighted with purple underline in each row
• Switch period tabs — sort selection persists across tab changes
• Check dark, light, and monokai themes

🤖 Generated with Claude Code

[vakovalskii]

Already implemented in v6.15.14 — sort by Messages/Hours/Cost added.