[site] Fix preview login redirects (#28555)

## Summary
- Normalize post-login redirect targets against the active Vite/router
basename.
- Prevent UI preview links from redirecting to duplicated paths like
`/preview/pr-28544/preview/pr-28544/`.
- Preserve the existing full-page redirect behavior for `/ops` paths,
including preview ops paths.

## Testing
- `yarn workspace @lightsparkdev/site exec vitest run
src/hooks/loginRedirect.test.ts`
- `yarn workspace @lightsparkdev/site exec eslint
src/hooks/useLoginAndRedirect.tsx src/hooks/loginRedirect.ts
src/hooks/loginRedirect.test.ts`
- `yarn workspace @lightsparkdev/site exec prettier --check
src/hooks/useLoginAndRedirect.tsx src/hooks/loginRedirect.ts
src/hooks/loginRedirect.test.ts`

## Notes
- Full `@lightsparkdev/site` typecheck is blocked locally by existing
unresolved workspace package exports such as `@lightsparkdev/ui/router`.

GitOrigin-RevId: d0319351285faf26145bf5d920525218a47fbcad

Author: coreymartin

packages/ui/package.json

@@ -83,9 +83,9 @@
     "./src/router": "./src/router.js"
   },
   "scripts": {
-    "build:bundle": "tsdown",
+    "build:bundle": "tsdown --config-loader unrun",
     "build": "yarn tsc && yarn build:bundle",
-    "build:watch": "tsdown --watch --no-clean",
+    "build:watch": "tsdown --config-loader unrun --watch --no-clean",
     "circular-deps": "madge --circular --extensions ts,tsx .",
     "clean": "rm -rf .turbo",
     "format:fix": "prettier . --write",