Fix advancement percentage slider to skip invalid range below champions minimum

[Copilot]

Description

Champions award winners always advance, so the slider previously allowed setting percentages that would advance fewer teams than champions awards — an invalid state. The slider now jumps directly from 0 (disabled) to the minimum percentage that covers all champions awards, then slides freely above that.

Changes

apps/admin/.../event-settings-section.tsx

• Fetch awards for all divisions in parallel to count champions per division
• Compute minAdvancementPercent: for each division, the smallest integer p where Math.round(teamCount × p / 100) ≥ championsCount, taking the max across divisions
  • Formula: Math.ceil((100 × champCount − 50) / teamCount) (derived from Math.round(x) ≥ c ⟺ x ≥ c − 0.5)
• Slider onChange snaps values in (0, minPercent) based on drag direction: dragging up → minPercent, dragging down → 0
• Adds a tick mark at minAdvancementPercent (deduplicated against existing 25/50/75/100 marks)

if (minAdvancementPercent !== null && v > 0 && v < minAdvancementPercent) {
  setAdvancementPercent(v < advancementPercent ? 0 : minAdvancementPercent);
}

Type of change

• Bug fix (non-breaking change which fixes an issue)

Screenshots

UI change: slider now has a visible mark at the minimum valid non-zero percentage and skips over the invalid range.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• My changes generate no new warnings