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.