fix: add nil fallback in get_main_width and fix width type annotation

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Co-authored-by: sand4rt <17591696+sand4rt@users.noreply.github.com>

Author: Copilot

lua/zen/init.lua

@@ -4,7 +4,7 @@
 --- @field filetype Filetype
 
 --- @class Config
---- @field main? { width?: number | fun(): number; }
+--- @field main? { width: number | fun(): number; }
 --- @field top? Integration[]
 --- @field right? { min_width?: number; [number]: Integration[]}
 --- @field bottom? Integration[]
@@ -17,9 +17,10 @@
 --- @field bottom Integration[]
 --- @field left { min_width: number; [number]: Integration[]}
 
+local default_width = 148
 --- @type ConfigOptions
 local opts = {
-	main = { width = 148 },
+	main = { width = default_width },
 	top = {},
 	right = { min_width = 46 },
 	bottom = {},
@@ -32,9 +33,12 @@ local state = {
 --- Resolves the configured main width.
 ---@return number
 local function get_main_width()
-	local width = opts.main.width
+	local width = opts.main and opts.main.width
 	if type(width) == "function" then
-		return width()
+		width = width()
+	end
+	if type(width) ~= "number" then
+		return default_width
 	end
 	return width
 end