feat(update): add version check to skip update when already latest

Now `nupst update` checks current version against latest release before
downloading anything.

Behavior:
- Fetches latest version from Gitea API
- Compares with current version
- Shows "Already up to date!" if versions match
- Only downloads/installs if newer version available

Example output when up to date:
  Checking for updates...
  Current version: v4.0.8
  Latest version:  v4.0.8

  ✓ Already up to date!

deno.json

@@ -1,6 +1,6 @@
 {
   "name": "@serve.zone/nupst",
-  "version": "4.0.7",
+  "version": "4.0.8",
   "exports": "./mod.ts",
   "tasks": {
     "dev": "deno run --allow-all mod.ts",