Reduce scheduler platform mapping conflicts and align posting with mock social API

[Jakedoes1111]

Motivation

• Avoid merge conflicts by keeping the platforms array shape unchanged and moving API naming to a small mapping helper.
• Ensure the scheduler does not rely on stale connectedAccounts state when deciding whether to post.
• Standardize scheduler publishing through the local mock social API and make JSON POSTs explicit for compatibility.
• Fix an incorrect LinkedIn lifecycle state returned by OAuth posting helpers.

Description

• Added platformApiNameMap and getApiPlatformName and kept the platforms array unchanged in components/ContentScheduler.tsx to reduce merge conflicts.
• Added fetchConnectedAccounts() and switched the scheduler to fetch latest accounts and post to the mock endpoint at ${socialApiBase}/publish using Content-Type: application/json.
• Replaced previous oauthService connectivity checks in the scheduler with checks against the freshly fetched accounts and mapping via getApiPlatformName.
• Ensured JSON POSTs in services/oauthService.ts include the Content-Type: application/json header and corrected LinkedIn lifecycleState to 'PUBLISHED'.

Testing

• No automated tests were run for these changes.

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Codex Task