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/config.rs

@@ -23,9 +23,11 @@ const DEFAULT_CONFIG_EMBEDDED: &str = include_str!("../default.config.yaml");
 
 #[derive(Debug, Deserialize)]
 pub struct Config {
-    pub server:   ServerConfig,
-    pub web:      WebConfig,
-    pub llm:      LlmConfig,
+    pub server:      ServerConfig,
+    pub web:         WebConfig,
+    pub llm:         LlmConfig,
+    #[serde(default)]
+    pub marketplace: MarketplaceConfig,
     #[serde(default)]
     pub tic:      TicConfig,
     #[serde(default)]
@@ -47,6 +49,23 @@ pub struct WebConfig {
     pub static_dir: String,
 }
 
+/// The connector marketplace feed (blueprint §14/§15).
+///
+/// Configurable, not hardcoded: an on-premise product must not hard-require
+/// reaching one vendor's host. Point it at a self-hosted mirror, or an offline
+/// copy served locally, and nothing else changes.
+#[derive(Debug, Deserialize)]
+pub struct MarketplaceConfig {
+    /// Base URL serving `connectors.json` and each `<folder>/connector.json`.
+    pub url: String,
+}
+
+impl Default for MarketplaceConfig {
+    fn default() -> Self {
+        Self { url: "https://connectors.skaldagent.net".to_string() }
+    }
+}
+
 impl Config {
     pub fn into_split(self) -> (skald_core::config::CoreConfig, crate::frontend::config::FrontendConfig) {
         let tz = self.timezone.clone();
@@ -58,9 +77,10 @@ impl Config {
                 timezone: self.timezone,
             },
             crate::frontend::config::FrontendConfig {
-                server:   self.server,
-                web:      self.web,
-                timezone: tz,
+                server:      self.server,
+                web:         self.web,
+                marketplace: self.marketplace,
+                timezone:    tz,
             },
         )
     }