Redesign import page: file-browser UX with rich preview

- Add `files` and `manifest` to CompanyPortabilityPreviewResult so the
  import UI can show actual file contents and metadata
- Rewrite import preview as a file/folder tree (matching export page
  design language) with per-file checkboxes to include/exclude items
- Show action badges (create/update/skip) on each file based on the
  import plan, with unchecked files dimmed and badged as "skip"
- Add rich frontmatter preview: clicking a file shows parsed frontmatter
  as structured data (name, title, reportsTo, skills) plus markdown body
- Include skills count in the sidebar summary
- Update import button to show dynamic file count that updates on
  check/uncheck
- Both /tree/ and /blob/ GitHub URLs already supported by backend

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/shared/src/types/company-portability.ts

@@ -186,6 +186,8 @@ export interface CompanyPortabilityPreviewResult {
     projectPlans: CompanyPortabilityPreviewProjectPlan[];
     issuePlans: CompanyPortabilityPreviewIssuePlan[];
   };
+  manifest: CompanyPortabilityManifest;
+  files: Record<string, string>;
   envInputs: CompanyPortabilityEnvInput[];
   warnings: string[];
   errors: string[];