feat(files): add space header extension point

[flash7777]

Summary

Add app.files.space-header extension point (type customComponent) that allows extensions to replace the default SpaceHeader on project space frontpages with a custom component.

Motivation

Folder view extensions (e.g. typed folder views, DMS views) need to render their own space header with domain-specific information (icons, metadata, action buttons). Currently the only option is to modify GenericSpace.vue in the core, which creates maintenance burden and tight coupling.

This extension point follows the same pattern as the existing app.files.sidebar.* and app.files.folder-views.* extension points.

Changes

File	Change
extensionPoints.ts	Add spaceHeaderExtensionPoint definition
GenericSpace.vue	Check for registered extension; render CustomComponentTarget if present, otherwise default SpaceHeader

How extensions use it

// In an extension's setup():
extensions: computed(() => [
  {
    id: 'my-extension.space-header',
    type: 'customComponent',
    extensionPointIds: ['app.files.space-header'],
    content: markRaw(MySpaceHeaderComponent),
    componentProps: () => ({ space, resources })
  }
])

Backward compatibility

No breaking changes. When no extension registers at this point, the default SpaceHeader renders exactly as before.

[JammingBen]

I'd rather make this extension point a bit more generic by removing the check on isSpaceFrontpage. This way, it would not only go for project spaces, but for all generic spaces. You can then further specify in your extension when it should be displayed.

And then maybe rather name the extension point app.files.generic-space-header.

[JammingBen]

Please don't hide this if a generic space header extension was found. There might be extensions were you actually want both.

We don't have a final solution for this yet, but for the time being, you could hide the space header in your custom extension via CSS.

[JammingBen]

This was just missing a format and a rebase (same as your other PR #2726).

The last commit 1e3a9d0 I don't quite like. The approach will become an issue as soon as genericSpaceHeaderExtensionPoint allows multiple extensions (which I think will/should be the case in the future). Because then the fallback might get rendered multiple times.

What were you missing without that commit?