LinkedIn: personal vs company-page posting targets #41

Part of #38 — Social account linking (OAuth) and full cross-post destination support

Web behaviour (/help/cross-posting → Choosing where a post goes on LinkedIn): a post can
go to the user's personal LinkedIn feed and/or to company pages they administer.

POST /api/messages accepts linkedInTargets[] — each entry { kind: "personal" } or
{ kind: "orgPage", pageId } — plus linkedInLinkAsFirstComment (post the link as the first
comment instead of in the body). Android sends only the boolean crossPostToLinkedIn, which is
the superseded form.

• Fetch targets with GET /api/linkedin/posting-targets / GET /api/linkedin/targets; refresh
  the page list with POST /api/linkedin/sync-pages; persist the user's default selection with
  PUT /api/linkedin/posting-targets.
• Add the destination picker to the composer and to the scheduled-post editor, as the web
  does.
• Add the link-as-first-comment toggle (only meaningful when the content contains a URL).

Tests: targets parse; composer sends linkedInTargets[]; first-comment toggle round-trip.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.