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.

README.md

@@ -1,7 +1,12 @@
 # live-server.nvim
 
-Live reload HTML, CSS, and JavaScript files inside Neovim with the power of
-[live-server](https://www.npmjs.com/package/live-server).
+Live reload HTML, CSS, and JavaScript files inside Neovim. No external
+dependencies — the server runs entirely in Lua using Neovim's built-in libuv
+bindings.
+
+## Dependencies
+
+- Neovim >= 0.10
 
 ## Installation
 
@@ -12,12 +17,17 @@ Install with your package manager or via
 luarocks install live-server.nvim
 ```
 
-## Dependencies
-
-- [live-server](https://www.npmjs.com/package/live-server) (install globally via npm/yarn)
-
 ## Documentation
 
 ```vim
 :help live-server.nvim
 ```
+
+## Known Limitations
+
+- **No recursive file watching on Linux**: libuv's `uv_fs_event` only supports
+  recursive directory watching on macOS and Windows. On Linux (inotify), the
+  `recursive` flag is silently ignored, so only files in the served root
+  directory trigger hot-reload. Files in subdirectories (e.g. `css/style.css`)
+  will not be detected. See
+  [libuv#1778](https://github.com/libuv/libuv/issues/1778).