fix(reload): bind SSE server to port 0 for OS-assigned port

Problem: the SSE reload server hardcoded port 5554, causing silent
failure when that port was already in use. bind() would fail but its
return value was never checked; listen() would also error and silently
drop via the if err then return end guard. inject() still wrote the
dead EventSource URL into the HTML, so the browser would connect to
whatever was on 5554 — or nothing — and live reload would silently
stop working.

Solution: bind to port or 0 so the OS assigns a free port, then call
getsockname() after bind to capture the actual port into actual_port.
inject() reads actual_port in preference to the hardcoded constant,
and stop() resets it. PORT = 5554 is kept only as a last-resort
fallback in inject() if actual_port is unset.

lua/preview/reload.lua

@@ -2,6 +2,7 @@ local M = {}
 
 local PORT = 5554
 local server_handle = nil
+local actual_port = nil
 local clients = {}
 
 local function make_script(port)
@@ -14,12 +15,15 @@ local function make_script(port)
 end
 
 function M.start(port)
-  port = port or PORT
   if server_handle then
     return
   end
   local server = vim.uv.new_tcp()
-  server:bind('127.0.0.1', port)
+  server:bind('127.0.0.1', port or 0)
+  local sockname = server:getsockname()
+  if sockname then
+    actual_port = sockname.port
+  end
   server:listen(128, function(err)
     if err then
       return
@@ -66,6 +70,7 @@ function M.stop()
     server_handle:close()
     server_handle = nil
   end
+  actual_port = nil
 end
 
 function M.broadcast()
@@ -85,7 +90,7 @@ function M.broadcast()
 end
 
 function M.inject(path, port)
-  port = port or PORT
+  port = actual_port or port or PORT
   local f = io.open(path, 'r')
   if not f then
     return