build: manage pnpm with mise at latest

Drop .mise.toml (pinned pnpm 12.3.4 + node 26; .mise.toml also shadows
mise.toml) and the packageManager pin; mise.toml tracks pnpm latest.

.mise.toml

@@ -1,3 +0,0 @@
-[tools]
-node = "26"
-pnpm = "12.3.4"

mise.toml

@@ -0,0 +1,2 @@
+[tools]
+pnpm = "latest"

package.json

@@ -2,7 +2,6 @@
   "name": "ddcbrightness",
   "version": "0.1.1",
   "description": "A GNOME Extension to control display brightness via DDC/CI using ddcutil",
-  "packageManager": "pnpm@12.3.4",
   "type": "module",
   "private": true,
   "author": "Lucas Oskorep <lucas.oskorep@gmail.com>",