fix: move api-contract to devDependencies to unbreak global install (#225)

`@webmux/api-contract` was listed as a runtime dependency with
`workspace:*`, which fails to resolve when users run
`bun install -g webmux@latest` outside the monorepo. The package is
already inlined into the bundled outputs by `bun build`, so it only
needs to be a devDependency for local workspace resolution.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: centdix

package.json

@@ -24,9 +24,7 @@
     "frontend",
     "packages/*"
   ],
-  "dependencies": {
-    "@webmux/api-contract": "workspace:*"
-  },
+  "dependencies": {},
   "scripts": {
     "dev": "bash dev.sh",
     "start": "bun bin/webmux.js",
@@ -41,6 +39,7 @@
     "frontend/dist/"
   ],
   "devDependencies": {
+    "@webmux/api-contract": "workspace:*",
     "@clack/prompts": "^1.1.0",
     "@sveltejs/vite-plugin-svelte": "^5.0.0",
     "@tailwindcss/vite": "^4.2.0",