TUI Plugin sharing - add remote plugin section plumbing

[canvrno-oai]

This adds the background plumbing for remote-backed plugin catalog sections while leaving the fuller directory presentation to the next PR. The TUI can fetch section-specific remote marketplace results, keep local plugin data available, and carry section errors forward for later rendering.

• Fetches explicit remote marketplace kinds for curated, workspace, and shared-with-me sections.
• Gates shared-with-me loading on the plugin sharing feature flag.
• Adds section-level error state and user-actionable error copy.
• Merges remote marketplace results into the cached plugin list without discarding local results.
• Validation: not run; this was a branch split/publish pass.