fix(editor): make page slug editable via a new Page settings dialog

[gxvr]

A page's slug could only be set at creation time — the site explorer's inline rename only ever updates the title (renamePage(id, title) with no third arg leaves the slug untouched), and there was no other slug editor for a regular (non-template) page. The Data tab correctly locks slug as an editor-managed built-in field, but the editor had nowhere to actually manage it.

Add a "Page settings" context-menu item (regular pages only - template pages already get title+slug through the existing Template settings dialog) that opens a small Title + Slug form, reusing the same slug validation helpers as TemplateSettingsDialog. The homepage's slug field is disabled since "index" is a reserved sentinel, not a freely editable value.

Extracted the Template/Page settings dialog state and handlers into a usePageSettingsDialogs hook — adding the new dialog inline pushed SiteExplorerPanel.tsx over the 700-line module-size-budgets ceiling.

Summary

Describe the change and why it is needed.

Verification

• bun run build
• bun test
• bun run lint
• Docker/deployment check, if relevant

Checklist

• Tests cover behavior changes.
• Docs were updated when behavior, config, deployment, or public surfaces changed.
• No compatibility shim was added for old pre-release behavior.
• No secrets, local databases, uploads, or generated artifacts are included.