feat(cc-config): manage plugins, marketplaces, hooks and settings from the dashboard

The Claude Config page was read-only for these four surfaces. It can now:

- register a marketplace from a pasted GitHub `owner/repo`, http(s) URL or
  absolute path, and install / enable / disable / update / uninstall plugins,
  all delegated to the `claude plugin …` CLI so its registry stays consistent;
- edit hooks and settings.json keys in the user, project and project-local
  layers.

Server:

- `server/lib/cc-plugin-cli.js` wraps the CLI with `execFile` and an explicit
  argv array — never a shell string. Every argument is validated before the
  spawn (shape allowlist per kind, no control characters, no leading `-`, no
  `..`), `CLAUDE_CONFIG_DIR` is pinned to the dashboard's CLAUDE_HOME, stdin is
  closed so a prompt errors instead of hanging, and mutating calls are
  serialized in-process.
- `cc-mutate.patchSettings` merges individual dotted keys into one settings
  layer. It re-reads the file unredacted, so keys the UI never displayed —
  including secret-valued ones — survive byte-for-byte. A payload carrying the
  `<redacted>` placeholder is refused (EREDACTED), as are `__proto__` /
  `prototype` / `constructor` segments; a settings file that does not parse is
  reported (EBADJSON) rather than clobbered. Backup first, then atomic write.
- Six additive routes on /api/cc-config; a non-zero CLI exit is a 422 carrying
  the CLI's own message, a missing binary a 503.

Client: install/add bars, per-card plugin and marketplace actions behind a
two-click confirm, and edit modes for hooks and settings that send only the
keys that changed. New i18n keys in en + vi; the explainer banners that said
these surfaces were read-only now describe what the writes actually do.

Tests: 61 new server tests (CLI argv construction and validation, the settings
key-merge, and the six routes against a fake `claude` on PATH).

docs/PLUGINS.md

@@ -426,7 +426,7 @@ These plugins query the Agent Monitor API at `http://localhost:4820`. Key data s
 Rule types: `token_threshold` (`{ total_tokens }` — the spend-relevant guardrail), `event_pattern`, `inactivity`, `status_duration`.
 
 ### Config Explorer (`/api/cc-config/*`)
-Read every Claude Code surface (skills, agents, commands, output-styles, plugins, marketplaces, mcp, hooks, settings, keybindings, statusline, memory). `memory` includes the per-project file-based store with `scope: "auto-memory"` (carrying `project`, `name`, `isIndex`, `frontmatter`); those files plus `CLAUDE.md` are mutable via `PUT`/`DELETE /api/cc-config/file` with always-on timestamped backups.
+Read every Claude Code surface (skills, agents, commands, output-styles, plugins, marketplaces, mcp, hooks, settings, keybindings, statusline, memory). `memory` includes the per-project file-based store with `scope: "auto-memory"` (carrying `project`, `name`, `isIndex`, `frontmatter`); those files plus `CLAUDE.md` are mutable via `PUT`/`DELETE /api/cc-config/file` with always-on timestamped backups. The page also **manages plugins and marketplaces**: paste a GitHub `owner/repo`, URL or local path to register a marketplace, then install / enable / disable / update / uninstall its plugins — each button runs the corresponding `claude plugin …` command on the dashboard host (argv array, never a shell string). Hooks and `settings.json` keys are editable per layer (user / project / project-local) through `PATCH /api/cc-config/settings`, a per-key merge that leaves every untouched key — including redacted secrets — exactly as it was.
 
 ## Plugin Development