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>

server/src/services/company-portability.ts

@@ -2064,6 +2064,8 @@ export function companyPortabilityService(db: Db) {
         projectPlans,
         issuePlans,
       },
+      manifest,
+      files: source.files,
       envInputs: manifest.envInputs ?? [],
       warnings,
       errors,