fix(ui): reset metadata dirty state

[perber]

Keep metadata-only saves from staying dirty after a successful save. Preserve the two-column metadata layout when only properties exist. Add E2E coverage for saving only properties and removing all properties.

[Copilot]

Pull request overview

This PR fixes the editor’s “dirty” state handling for metadata-only saves by ensuring the local editor snapshot is updated canonically after a successful save, even when the API omits empty collections. It also adjusts viewer layout so properties-only metadata keeps the intended two-column alignment, and adds E2E coverage for these scenarios.

Changes:

• Sync editor store state (page.tags, page.properties, tags, and frontmatterFields) after save to prevent metadata-only saves from remaining dirty.
• Keep the viewer metadata container in two-column mode when properties exist (even without tags), with a standalone alignment tweak.
• Add Playwright E2E tests for saving properties-only metadata and removing all properties.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
ui/leafwiki-ui/src/index.css	Adds a grid column-start rule to align properties when they’re the only metadata shown in two-column layout.
ui/leafwiki-ui/src/features/viewer/PageMetadata.tsx	Applies two-column metadata layout when properties exist and adds a standalone class when tags are absent.
ui/leafwiki-ui/src/features/editor/pageEditorStore.ts	Canonicalizes local tags/properties/frontmatter fields after save to correctly reset dirty state for metadata-only updates.
e2e/tests/editor.spec.ts	Adds E2E coverage for properties-only saves and removing all properties (verifying UI + API state).

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