Avoid stale connected account fallback; post via mock social API and add JSON headers

[Jakedoes1111]

Motivation

• Prevent the scheduler from using potentially stale connectedAccounts state when the mock accounts endpoint cannot be reached.
• Standardize scheduler publishing on the local mock social API instead of the partial OAuth client in UI code.
• Ensure all JSON POSTs include the appropriate Content-Type: application/json header for API compatibility.
• Fix an incorrect LinkedIn lifecycle state value returned by the OAuth posting helpers.

Description

• Updated components/ContentScheduler.tsx to add a SocialAccount interface, socialApiBase, and fetchConnectedAccounts() which returns an empty list on failure instead of falling back to stale state.
• Switched scheduler posting to the mock endpoint at /api/social/publish by mapping UI platform ids to apiName and sending JSON payloads with Content-Type: application/json.
• Removed reliance on oauthService.isConnected in the scheduler and now check connectivity against the freshly fetched accounts before posting.
• Modified services/oauthService.ts to add Content-Type: application/json headers to JSON POSTs and corrected LinkedIn lifecycleState from 'pUBLISHED' to 'PUBLISHED'.

Testing

• No automated tests were run for these changes.

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Codex Task