feat: replace npm live-server with pure-Lua HTTP server (#29)

## Problem

The plugin requires users to install Node.js and the `live-server` npm
package globally. This is a heavyweight external dependency for what
amounts to a simple local dev-server workflow, and it creates friction
for users who don't otherwise need Node.js.

## Solution

Replace the npm shell-out with a pure-Lua HTTP server built on `vim.uv`
(libuv bindings), eliminating all external dependencies. The new server
supports static file serving, SSE-based live reload, CSS hot-swap
without full page reload, directory listings, and recursive file
watching with configurable debounce.

Minimum Neovim version is bumped to 0.10 for `vim.uv` and `vim.ui.open`.
The old `args`-based config is automatically migrated with a deprecation
warning.

Closes #28.

.pre-commit-config.yaml

@@ -14,4 +14,4 @@ repos:
     hooks:
       - id: prettier
         name: prettier
-        files: \.(md|toml|ya?ml|sh)$
+        files: \.(md|toml|yaml|yml|sh)$