feat(plugin): marketplace — install plugins from a curated registry

Add a Plugin Manager marketplace that installs external add-ons from a
GitHub repo's Releases. A curated registry (plugins/registry.json in this
repo, extended by PR) lists discoverable plugins; the host fetches it,
shows each entry with a release dropdown, and on install downloads the
asset matching the user's platform plus plugin.toml into the plugins
folder (API-version checked first). Users can also link arbitrary
owner/repo manually. Linked repos persist in settings.

Desktop only (ureq + serde_json); the install path reuses the phase-2
loader. Part of the #100 extensibility epic (phase 2).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -81,6 +81,8 @@ open = "5"
 ureq = { version = "3", default-features = false, features = ["rustls"] }
 # Runtime loading of external plugin cdylibs (phase 2, desktop only).
 libloading = "0.8"
+# Parse the GitHub Releases API response for the plugin marketplace.
+serde_json = "1"
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 console_error_panic_hook = "0.1"