feat(ui): declutter dashboard done-recently + MCP-access, add per-project stats

Dashboard:
- 'Done recently' chip-cloud -> compact uniform list (Active-now row style),
  showing 5 with inline expand to the rest (backend already returns up to 8).
- New 'Projects' rail card: each active project with 'N open · M done'.
  Backend already computed done_count (dashboard.py) — now surfaced in the
  /api/dashboard payload per active project.

MCP Access (Connect Claude / Claude Code):
- Progressive disclosure: lead with the pre-filled plugin-install snippet;
  fold server name, scope, marketplace URL, and the MCP-only path into a
  single 'Customize' expander. Desktop tab keeps its own server-name field.
- Marketplace URL now defaults to this instance's own repo via
  config.PLUGIN_MARKETPLACE_URL (env-overridable); /api/plugin/marketplace-url
  falls back to it, so the field + install snippet are pre-filled out of the
  box instead of showing a generic placeholder.

Refs #761

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/scribe/routes/plugin.py

@@ -11,6 +11,7 @@ from __future__ import annotations
 from quart import Blueprint, g, jsonify, request
 
 from scribe.auth import admin_required, get_current_user_id, login_required
+from scribe.config import Config
 from scribe.services import plugin_context as plugin_ctx_svc
 from scribe.services import repo_bindings as repo_bindings_svc
 from scribe.services.settings import get_admin_setting, set_setting
@@ -60,8 +61,10 @@ async def session_context():
 @login_required
 async def get_marketplace_url():
     """The plugin marketplace git URL, readable by any logged-in user so their
-    Settings page can show a copyable install command."""
-    return jsonify({"marketplace_url": await get_admin_setting(_MARKETPLACE_KEY)})
+    Settings page can show a copyable install command. Falls back to the
+    config default (this deployment's own repo) when no admin override is set."""
+    url = await get_admin_setting(_MARKETPLACE_KEY, default=Config.PLUGIN_MARKETPLACE_URL)
+    return jsonify({"marketplace_url": url})
 
 
 @plugin_bp.put("/marketplace-url")