feat(dashboard): add Plugins page with enable/disable, auth status, install/remove

- New PluginsPage.tsx: full plugin management UI (list, enable/disable,
  install from git, remove, git pull updates, provider picker)
- Backend: dashboard_set_agent_plugin_enabled now also toggles the
  plugin's toolset in platform_toolsets so enabling actually makes
  tools visible in agent sessions
- Backend: /api/dashboard/plugins/hub returns auth_required + auth_command
  per plugin (checks tool registry check_fn)
- Frontend: auth_required shown as Badge + CommandBlock with copy-able
  auth command
- Fix: Select overflow in providers card (min-w-0 grid cells, removed
  truncate/overflow-hidden that clipped dropdown)
- Refactor: _install_plugin_core extracted for non-interactive reuse,
  PluginOperationError for structured error handling
- i18n: en/zh/types updated with all new plugin page strings

web/src/plugins/slots.ts

@@ -46,6 +46,8 @@ import React, { Fragment, useEffect, useState } from "react";
  *  - `cron:bottom`      — bottom of /cron page
  *  - `skills:top`       — top of /skills page
  *  - `skills:bottom`    — bottom of /skills page
+ *  - `plugins:top`       — top of /plugins page
+ *  - `plugins:bottom`    — bottom of /plugins page
  *  - `config:top`       — top of /config page
  *  - `config:bottom`    — bottom of /config page
  *  - `env:top`          — top of /env (Keys) page
@@ -78,6 +80,8 @@ export const KNOWN_SLOT_NAMES = [
   "cron:bottom",
   "skills:top",
   "skills:bottom",
+  "plugins:top",
+  "plugins:bottom",
   "config:top",
   "config:bottom",
   "env:top",