feat(soccer-app): add shared team metrics utilities

[ayushtr-aws]

Summary

Introduces reusable, typed soccer team metric helpers in a new soccer-app/src/utils/teamMetrics.ts module (TG-10). No UI changes — this only adds utilities for future consumption.

Exported helpers:

• goalDifference(stats) — goals scored minus goals conceded (signed).
• winRate(stats) — wins as a fraction of matches played, in [0, 1], rounded to 4 decimals for deterministic output. Returns 0 when no matches have been played (safe zero-division handling).
• performanceLabel(stats) — a concise PerformanceLabel ("Elite" | "Strong" | "Average" | "Struggling") derived from the win rate. Teams with zero matches get a neutral "Average" default.

All helpers accept the existing TeamStats type from src/data/teams.ts and are pure/deterministic.

Task

TG-10: Add shared team metrics utilities (sub-issue of epic "ABCA Jira stacked corrections acceptance round 2").

Build & test results

Ran from soccer-app (dependencies installed with npm ci first, as node_modules was absent):

• mise run build (tsc -b && vite build) — PASS: 20 modules transformed, built successfully.
• mise run lint (oxlint) — PASS: no warnings or errors (exit 0).

The repository has no test suite; validation was done via the TypeScript compiler (strict noUnusedLocals/noUnusedParameters) and the linter.

Decisions made

• Placed the module at the exact path required (soccer-app/src/utils/teamMetrics.ts) and reused the existing TeamStats interface rather than defining a new shape, to stay consistent with the codebase.
• Chose win-rate-based thresholds for performanceLabel (0.7 / 0.5 / 0.3) as a simple, deterministic mapping. Zero-match teams return "Average" and winRate returns 0 to handle division safely.
• Rounded winRate to 4 decimal places to keep outputs deterministic.
• Used import type to comply with the project's verbatimModuleSyntax setting.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.

Agent notes

• What went well: The task was concrete and self-contained. The existing TeamStats interface made typing straightforward.
• What was difficult: soccer-app/node_modules was not installed despite the setup step reporting mise install: OK — tsc/vite/oxlint binaries were missing, so npm ci had to be run inside soccer-app before the build/lint tasks would work.
• Repo conventions discovered: The app lives under soccer-app/; build is tsc -b && vite build, lint is oxlint (config in .oxlintrc.json). mise.toml at the root proxies build/lint/dev tasks into the soccer-app dir. TS config enables verbatimModuleSyntax, so type-only imports must use import type.
• Suggestions for future tasks: Consider adding a test runner (e.g. Vitest) — there is currently no test suite, so utility correctness can only be verified via the compiler/linter. Ensuring soccer-app deps are pre-installed in the environment would also speed up build/lint verification.