fix: leaderboard columns going stale when context is shown for all measures

[nishantmonu51]

• The leaderboard's colgroup, header and rows each called a locally declared shouldShowContextColumns() to decide which context columns to render. In Svelte 5 legacy mode that call only re-evaluates when the identifiers in the expression change, so the prop it reads is never tracked and the block goes stale until the component is remounted.
• Toggling "show context for all measures" with a time comparison active therefore updated the rows (they remount when the query refetches) but not the colgroup or the header: the body rendered delta columns the table never declared, measure labels slid onto the previous measure's delta columns, and the surplus cells collapsed into overlapping text at the right edge. A page reload rendered it correctly, which is why it only showed up after toggling.
• Leaderboard.svelte now derives measuresWithContext reactively and passes it to LeaderboardHeader and LeaderboardRow, so all three stay in sync.
• Also fixes columnCount, which counted percent-of-total from the sort measure only, and tableWidth, which only ever counted a single measure's columns.
• Added LeaderboardHeader.spec.ts, which fails against the old function-based header (one delta column instead of two) and passes now.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!