feat: add platform-aware previews to the Socials publish pane

[chubes4]

Dependency

Blocked by Extra-Chill/data-machine-socials#140, which must expose the platform presentation/configuration needed for accurate previews.

Current state

The publish pane already owns caption text, selected media, ordering, platform selection, image limits, character limits, and direct publishing. It has no platform-shaped preview.

Implementation owner:

• src/blocks/studio/tabs/socials/publish/index.ts

Goal

Add the smallest preview UI required by this real consumer. Do not build a standalone generic preview framework first.

Preview requirements

• reflect selected platform/account
• apply platform character and media constraints
• show image/carousel ordering
• provide explicit approximation language where exact rendering is impossible
• remain responsive beside or below the composer
• update from the same caption/media state used for publishing

Acceptance criteria

• Supported platform config comes from Data Machine Socials rather than Studio literals.
• Preview and publish consume one state source.
• Unsupported platforms degrade to a clear generic summary.
• Mobile and desktop layouts remain usable.
• Tests cover platform switching, character overflow, media ordering, no-media state, and unsupported preview config.
• Shared extraction happens only if another concrete consumer emerges.