chore: add session auth to event-board example

[dangreen]

Brings the event-board example up to a complete fullstack reference:

• in-memory API: login/logout/me, HttpOnly session cookie, per-user RSVP toggle, author on created events
• injectable ApiService$/EventsService$/UserService$/IntlService$; SSR requests forward the session cookie and locale
• login page, user zone in the header, route guard with SSR 302, localized 404 page
• svelte variant gets intl parity and is renamed to svelte-nano_kit-intl-ssr
• examples sync merges shared dirs instead of replacing them, keeping variant-local files
• .env.example per variant; API spec + auth integration tests (107 total)
• vite is deduped to a single 8.1.2 across the workspace

Depends on #166 — the 404 status integration test needs the notFound(loadable(...)) router fix; merge #166 first and update this branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.86%. Comparing base (2febcf7) to head (fa7cecd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #167   +/-   ##
=======================================
  Coverage   83.86%   83.86%           
=======================================
  Files         138      138           
  Lines        2969     2969           
  Branches      560      560           
=======================================
  Hits         2490     2490           
  Misses        340      340           
  Partials      139      139           

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