feat: remove page-level loaders from AppShellPageProps

[IzumiSy]

Summary

This PR removes loader from file-based page definitions (Page.appShellPageProps) and makes guards the only source of page-level route behavior in file-based routing. As part of that cleanup, route conversion now derives loaders only from guards, and the Vite plugin schema/validator now rejects loader on appShellPageProps. The docs and changeset have been updated to reflect the breaking change.

Why

loader on file-based pages was accidentally exposed but incomplete:

• it had no supported way to pass loader data into the page component
• it conflicted with guards, because an explicit page loader took precedence over the guard-backed loader
• it was also a mismatch with the programmatic API: Module / Resource carry an internal loader for guard wiring, but defineModule / defineResource do not expose loader as a public input prop
• removing it restores one consistent model: page-level route behavior comes from guards, not ad hoc loaders

Guard behavior

• pages using only guards: no behavior change
• pages using guards + loader: guards now run consistently because the page loader is no longer read

This means the guard story is now simpler and more predictable: if a page declares guards, those guards are what control redirect/404 behavior.

Practical impact

Based on a survey of existing projects, we did not observe current usage of page-level loader in AppShellPageProps, so the practical migration impact should be negligible.