feat(mcp): connector marketplace + split the Connectors surface (§7/§14/§15)

Fills a gap the blueprint names: the admin had to hand-author every
`mcp_catalog` entry. A remote feed of vetted connectors now proposes them
and the admin installs — the feed is *consultative*, so §14's risk axis is
untouched and the trust anchor stays on the box.

Marketplace client (`src/frontend/api/marketplace.rs`):
- Fetches the feed server-side (it sends no CORS headers) and caches it;
  icons are proxied for the same reason.
- Verifies every declared SHA-256 before writing, fail-closed and
  all-or-nothing. Feed-supplied paths are refused if they escape
  `./scripts/<id>/`. Importing an `mcp_local` entry still demands the
  admin-only `mcp.register_local_script`.
- Translates the feed's vocabulary into Skald's: `user`→`per_user`,
  `mcp_local`→`local_script`. Scope is read, never inferred from transport
  (a remote connector can be per-user — that is what `mcp.register_remote`
  is for), and an unreadable `type` fails closed to the answer needing more
  authority. The feed's `llm_short_description` maps to `description`, the
  column `render_mcp_list` puts in front of the LLM for `activate_tools()`.
- Feed URL is config (`marketplace.url`), not a constant: an on-premise
  product must not hard-require reaching one vendor's host.

Two silent failures found while wiring it:
- `transport_of` maps anything unknown to Stdio, so the feed's
  `streamable-http` would have tried to spawn a command. Normalised on import.
- Some servers want their key as a query param, not a bearer header, and say
  so with a `{key}` placeholder. Substituted at connect time in
  `global_row_spec`/`user_row_spec` — never at rest, so the key stays in its
  own column and the stored URL stays a template.

Pages, split by the question each answers:
- Connectors — what runs (`UserMcpView` = global ∪ per-user) and what I can
  add. Same page for everyone; the admin just has more verbs. One Available
  list with the verb per row: `per_user`→Activate, `global`→Enable globally.
  Enabling a global is the admin's counterpart to activating a per-user one,
  so the catalog picker dropdown is gone — the entry comes from the row.
- Connector Catalog (admin) — what this box offers. One `Add connector`
  with two sources: marketplace first (vetted, hashed), manual second
  (unvetted by nature) — the order mirrors the trust model.
- Marketplace (admin) — reached from the catalog, not the sidebar: it is a
  destination of an action, not a place.

`available()` no longer returns `McpGlobalServerRow`: that row carries
`api_key` and this view now reaches every logged-in user. A slim `GlobalView`
crosses instead, and an admin sees every global (with `can_use` marking their
own) so one enabled for someone else stays manageable.

Also fixes `connectors-page` having no CSS rule at all — every sibling page
has one, so it never got `flex: 1` and left an empty column beside it.

src/frontend/api/mod.rs

@@ -13,6 +13,7 @@ pub mod image_generate_models;
 pub mod images;
 pub mod inbox;
 pub mod llm;
+pub mod marketplace;
 pub mod mcp;
 pub mod mcp_media;
 pub mod plugins;
@@ -130,6 +131,11 @@ pub fn router() -> Router<Arc<Skald>> {
         .route("/sessions/{session_id}/run-context", put(run_context::set_session_run_context))
         // MCP / Connectors (blueprint §14/§15)
         .route("/mcp/servers",                  get(mcp::list_servers))
+        // admin: the remote marketplace feed (consultative — installing is the
+        // admin's act, and it lands in the catalog below)
+        .route("/mcp/marketplace",              get(marketplace::list))
+        .route("/mcp/marketplace/install",      post(marketplace::install))
+        .route("/mcp/marketplace/{id}/icon",    get(marketplace::icon))
         // admin: catalog + globally-active connectors
         .route("/mcp/catalog",                  get(mcp::catalog_list).post(mcp::catalog_upsert))
         .route("/mcp/catalog/{id}",             delete(mcp::catalog_delete))