Add a "default content types" site setting to preselect sites for new content

Problem/Motivation

There's no way for a site to declare which content types it hosts. New content always falls back to current-site/user heuristics, so editors must manually pick the site even when a site is the obvious default for a content type.

Proposed resolution

• New sites_content site setting: per site, check the entity types/bundles it should host by default (initial_content_types_selection), plus a canonical_by_weight toggle.
• Refactor the inline initial-selection logic in the SitesSelection element into an InitialSitesSelection event resolved by priority-ordered subscribers: form-state/user-input (200) → current site (100) → content-type setting (50) → current-user fallback (0); first to set wins.
• New content of an opted-in type is preselected for that site; with canonical_by_weight, the lowest-weight opted-in site becomes canonical.