feat(static-pages): media picker + per-page modified indicator

Milestone 4 of the Static Pages Module sprint.

- heroImage/thumbnail now wire through the existing MediaPickerComponent
  (same media-field-row + "choose image" pattern as branding-section), not
  plain URL text alone. gallery stays a lightweight CSV field ("future
  ready" per the brief - no dedicated multi-upload UI this sprint).
- ProjectEditorFacade: add originalStaticPages, a narrow computed exposing
  the originally loaded/published staticPages snapshot (mirrors the facade's
  existing pattern of small single-purpose computeds).
- StaticPagesEditorComponent: isModified(page) diffs a page against its
  normalized original snapshot, reusing ContentManagementFacade.pages() for
  normalization rather than reimplementing it. Renders as an amber
  "unsaved changes" badge per page.

Draft/published status UI, publish/unpublish actions, and the plain-text
media fields landed already in M2; this milestone completes M4's remaining
scope (visual media picker + modified indicator) without duplicating that
work.

Gate: tsc --noEmit, npm test (57/57), arch:check, build all green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/project-editor/facade/project-editor.facade.ts

@@ -63,6 +63,8 @@ export class ProjectEditorFacade {
   readonly lastSavedAt = computed(() => this.state().lastSavedAt);
   readonly lastPublishedAt = computed(() => this.state().lastPublishedAt);
   readonly draftRestored = computed(() => this.state().draftRestored);
+  /** The originally loaded/published `staticPages` snapshot, for per-page modified-state comparison in the Static Pages editor. */
+  readonly originalStaticPages = computed(() => this.state().originalBootstrap?.staticPages ?? null);
   readonly validationIssues = computed(() => {
     const current = this.bootstrap();
     return current ? this.validator.validate(current) : [];