fix(web): pin packageManager, drop unused @vite-pwa/sveltekit dep

CI's web job runs bare `corepack enable && pnpm install --frozen-lockfile`
with no explicit pnpm version, so without a `packageManager` field corepack
can resolve a different pnpm than the one that generated the lockfile
(lockfileVersion 9, requires pnpm 9+) — that's what broke the first CI run.
Also drops @vite-pwa/sveltekit, left over from an earlier approach abandoned
in favor of the base vite-plugin-pwa plugin (see vite.config.ts) and no
longer imported anywhere.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/package.json

@@ -6,6 +6,7 @@
 	"engines": {
 		"node": ">=22"
 	},
+	"packageManager": "pnpm@9.15.9",
 	"scripts": {
 		"dev": "vite dev",
 		"build": "vite build",
@@ -21,7 +22,6 @@
 		"@sveltejs/adapter-static": "^3.0.10",
 		"@sveltejs/kit": "^2.63.0",
 		"@sveltejs/vite-plugin-svelte": "^7.3.0",
-		"@vite-pwa/sveltekit": "^1.1.0",
 		"eslint": "^10.11.0",
 		"eslint-config-prettier": "^10.1.8",
 		"eslint-plugin-svelte": "^3.23.0",