test(ui): run e2e against the minified vite build, not the dev server

The e2e booted the unminified Vite dev server, so production-minification bugs
were invisible to it — exactly how a minifier dropping a bare `void recenter`
reactive read slipped through. Build the app in mock mode and serve the minified
artifact via `vite preview` instead, so the smoke exercises the same bundle the
contour ships. Build to dist-e2e/ (gitignored) so it never clobbers the dist/ the
bundle-size gate measures, and with `--base /` so the SPA-fallback also boots a
subpath like /telegram/ (the production relative base needs the gateway's path
mapping, absent under a plain preview). All 118 specs pass against the build on
both engines, including the hint-recenter spec — confirming the hardened
dependency survives minification.

ui/.gitignore

@@ -1,5 +1,6 @@
 node_modules/
 dist/
+dist-e2e/
 .svelte-kit/
 *.tsbuildinfo
 test-results/