Support plain permalinks for REST URL construction

[epeicher]

Summary

Fixes #183. Several REST call sites built URLs by concatenating a path onto the injected restRoot (or a sibling usersUrl / insightsUrlBase / pluginsUrl / baseUrl key) and assumed the pretty-permalink shape (/wp-json/...). On sites configured with Settings -> Permalinks -> Plain (the WordPress default for fresh installs), rest_url() emits the query-string shape (?rest_route=/...); concatenating ?context=edit or ?per_page=10 onto that produced two ? separators, and the URL parser collapsed the whole path into the rest_route value, dropping the trailing query params and causing 404s.

What changed

• New src/rest-url.ts exports joinRestUrl(restRoot, path), which handles both shapes:
  • Pretty: appends the path to the URL pathname.
  • Plain: appends the path to the existing rest_route query value and merges any query params from the path as separate searchParams.
• Migrated every unsafe call site to use the helper:
  • src/posts-window/{rest,users-rest,user-edit-rest,user-edit-render,tags-cloud,categories-mindmap}.ts
  • src/content-graph/rest.ts
  • src/oauth-relay.ts
  • src/my-wordpress/rest.ts
  • src/desktop-files/preview.ts
  • src/desktop-files/rest.ts (the call() helper, so every placements / folders / associations endpoint inherits the fix; the visible symptom on Plain was desktop icons taking several seconds to appear because the files layer 404'd on /desktop-mode/v1/files/placements?folder=0 and fell back to a slower path).
  • src/plugins-window/rest.ts (list / activate / deactivate / delete; visible symptom was the Plugins window showing "Could not load plugins: No route was found matching the URL and request method.").
• 9 new unit tests pinning the helper's behavior on both permalink shapes (route-only, route+query, with/without trailing slashes, etc.).

PHP side already adapted to permalink structure via rest_url(), so no PHP changes were needed.

Test plan

• npm run lint clean
• tsc --noEmit clean
• npm run test:js green (1125/1125, +9 new)
• npm run build produces all bundles
• Manual on local dev with Settings -> Permalinks -> Plain:
  • Posts window paginates, filters by tag / category / author
  • Users window opens, User Edit window loads with ?context=edit, save round-trips
  • My WordPress window opens and lists entities
  • Content Graph loads user / term / comment stats
  • Desktop icons render immediately on boot (no slow fallback)
  • Plugins window lists installed plugins (no "No route was found" toast)
• Same flows still work on Post name permalinks (no regression).

[github-actions]

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