feat(championship): surface teammate H2H bars on drivers view (#23)

Add reusable TeammateH2H component and teammatePairs helper that ranks
intra-team battles by closeness, using existing hub teammate_wins data.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/test/h2h.test.ts

@@ -0,0 +1,132 @@
+import { describe, expect, it } from 'vitest'
+import { teammatePairs } from '../lib/h2h'
+import type { ChampHubDriver } from '../types'
+
+function driver(over: Partial<ChampHubDriver>): ChampHubDriver {
+  return {
+    driver_number: 1,
+    name_acronym: 'VER',
+    full_name: 'Max Verstappen',
+    team_name: 'Red Bull',
+    team_colour: '3671c6',
+    points: 200,
+    position: 1,
+    wins: 5,
+    podiums: 8,
+    poles: 4,
+    form: [],
+    cumulative: [],
+    teammate_wins: 9,
+    teammate_losses: 1,
+    ...over,
+  }
+}
+
+describe('teammatePairs', () => {
+  it('returns empty for no drivers', () => {
+    expect(teammatePairs([])).toEqual([])
+  })
+
+  it('skips teams with a single driver', () => {
+    const drivers = [
+      driver({ driver_number: 1, team_name: 'Red Bull' }),
+      driver({ driver_number: 44, name_acronym: 'HAM', team_name: 'Mercedes', points: 50 }),
+    ]
+    expect(teammatePairs(drivers)).toHaveLength(0)
+  })
+
+  it('pairs two-driver teams', () => {
+    const drivers = [
+      driver({ driver_number: 1, name_acronym: 'VER', points: 200, teammate_wins: 9, teammate_losses: 1 }),
+      driver({
+        driver_number: 11,
+        name_acronym: 'PER',
+        full_name: 'Sergio Perez',
+        points: 60,
+        teammate_wins: 1,
+        teammate_losses: 9,
+      }),
+    ]
+    const pairs = teammatePairs(drivers)
+    expect(pairs).toHaveLength(1)
+    expect(pairs[0].teamName).toBe('Red Bull')
+    expect(pairs[0].driverA.name_acronym).toBe('VER')
+    expect(pairs[0].driverB.name_acronym).toBe('PER')
+    expect(pairs[0].extraCount).toBe(0)
+    expect(pairs[0].closeness).toBe(8)
+  })
+
+  it('uses top two by points for teams with 3+ drivers', () => {
+    const drivers = [
+      driver({ driver_number: 1, name_acronym: 'VER', team_name: 'Red Bull', points: 200 }),
+      driver({
+        driver_number: 11,
+        name_acronym: 'PER',
+        team_name: 'Red Bull',
+        points: 60,
+        teammate_wins: 1,
+        teammate_losses: 9,
+      }),
+      driver({
+        driver_number: 99,
+        name_acronym: 'LAW',
+        full_name: 'Liam Lawson',
+        team_name: 'Red Bull',
+        points: 10,
+        teammate_wins: 0,
+        teammate_losses: 0,
+      }),
+    ]
+    const pairs = teammatePairs(drivers)
+    expect(pairs).toHaveLength(1)
+    expect(pairs[0].driverA.name_acronym).toBe('VER')
+    expect(pairs[0].driverB.name_acronym).toBe('PER')
+    expect(pairs[0].extraCount).toBe(1)
+  })
+
+  it('sorts by closest battle first', () => {
+    const drivers = [
+      driver({
+        driver_number: 1,
+        name_acronym: 'VER',
+        team_name: 'Red Bull',
+        points: 200,
+        teammate_wins: 10,
+        teammate_losses: 0,
+      }),
+      driver({
+        driver_number: 11,
+        name_acronym: 'PER',
+        team_name: 'Red Bull',
+        points: 60,
+        teammate_wins: 0,
+        teammate_losses: 10,
+      }),
+      driver({
+        driver_number: 4,
+        name_acronym: 'NOR',
+        team_name: 'McLaren',
+        team_colour: 'ff8000',
+        points: 160,
+        teammate_wins: 6,
+        teammate_losses: 5,
+      }),
+      driver({
+        driver_number: 81,
+        name_acronym: 'PIA',
+        full_name: 'Oscar Piastri',
+        team_name: 'McLaren',
+        team_colour: 'ff8000',
+        points: 140,
+        teammate_wins: 5,
+        teammate_losses: 6,
+      }),
+    ]
+    const pairs = teammatePairs(drivers)
+    expect(pairs).toHaveLength(2)
+    expect(pairs[0].teamName).toBe('McLaren')
+    expect(pairs[0].closeness).toBe(1)
+    expect(pairs[1].teamName).toBe('Red Bull')
+    expect(pairs[1].closeness).toBe(10)
+  })
+})