fix(server): 404 on out-of-range slide patch - #2704

Summary

• return 404 when the slide editor requests an index outside the current deck
• reuse the validated slide in both GET and POST handling
• prevent a stale editor save from dereferencing a missing slide and terminating the dev server
• complete the repository's plan 011 tracking

Tests

• pnpm build
• pnpm typecheck
• pnpm test (27 files, 229 tests)
• pnpm exec eslint packages/slidev/node/vite/loaders.ts
• manually verified valid GET returns 200, out-of-range GET and POST return 404, and the server remains responsive after the invalid POST