chore(deps): upgrade vite 6→8 and related packages

Vite 8 uses Rolldown (Rust bundler), cutting build time from ~41s to ~6s.
Fix readonly Array<> syntax in caption-items.ts for stricter Rolldown TS parsing.
Add .vercel to .gitignore.

Author: walterlow

.gitignore

@@ -53,3 +53,5 @@ crates/*/Cargo.lock
 .knip-report.json
 
 tmp
+
+.vercel