fix: declare fscreen runtime dependency

[xianzuyang9-blip]

Summary

• move fscreen from the vidstack package dev dependencies into runtime dependencies
• keep the existing @types/fscreen dev dependency for local TypeScript builds
• refresh the pnpm lockfile so consumers receive fscreen when installing vidstack

Fixes #1841.

Why

packages/vidstack/src/foundation/fullscreen/controller.ts and packages/vidstack/src/core/api/player-state.ts import fscreen. The published server bundle can therefore resolve fscreen at runtime in SSR/test environments, but the package manifest only listed it as a dev dependency.

Verification

• corepack pnpm install --lockfile-only
• corepack pnpm install --frozen-lockfile --ignore-scripts
• node -e "const p=require('./packages/vidstack/package.json'); if(!p.dependencies.fscreen) throw new Error('fscreen missing from dependencies'); if(p.devDependencies.fscreen) throw new Error('fscreen still in devDependencies'); console.log('manifest ok', p.dependencies.fscreen)"
• corepack pnpm --filter vidstack typecheck
• corepack pnpm --filter vidstack exec vitest --run src/foundation/fullscreen/controller.test.ts src/core/api/player-state.test.ts

The focused Vitest command matched and ran src/core/api/player-state.test.ts successfully; there does not appear to be a matching controller.test.ts file in this checkout.