fix(vidstack): type legacy vite plugin

[mihar-22]

Summary

• Type the legacy Vite plugin helper as Vite's Plugin type.
• Keeps the enforce: 'pre' value checked against Vite's plugin contract.

Fixes #1792

Validation

• pnpm install --frozen-lockfile succeeded in the isolated fix/1792-vite-enforce-type worktree.
• pnpm exec tsgo -p tsconfig.json --noEmit failed at the repository root with exit code 2 and 1035 diagnostic lines. The failures are broad existing root tsconfig issues: React TSX files are checked without jsx, Vitest globals are missing for test files, and packages/vidstack/tsdown.config.ts cannot find rolldown types. No diagnostics matched packages/vidstack/vite.config.ts, legacyPlugin, or enforce.
• cd packages/vidstack && pnpm exec tsgo -p tsconfig.json --noEmit passed.