feat(security): full A13-3 open-redirect validation in postLoginRedirect

Replaces the WS-3 PR-B2a minimum precaution (`startsWith('/') &&
!startsWith('//')`) with a layered validator that rejects every input
that is not a strict relative path.

isSafeRelativePath rejects:
- Empty / null / undefined input
- Non-`/`-prefixed paths (including leading whitespace)
- Protocol-relative URLs (`//evil.com`)
- Backslash anywhere (browsers normalise `\` → `/` in some contexts;
  `/\evil.com` parses as `//evil.com`)
- ASCII control characters `\x00`–`\x1F` and `\x7F` (NUL, tab, LF, CR,
  DEL, etc. — header-injection vectors)
- Anything the URL constructor parses to a different origin than the
  synthetic invalid origin used as the resolution base

The URL-constructor check is the authoritative guard; the prefix and
character checks are fast pre-filters that short-circuit common
attack shapes without paying the URL allocation.

Test coverage expands from 6 → 16 cases. New cases pin the
backslash, control-character, leading-whitespace, and positive-
character-set contracts. The URL-encoded-slash-in-query case
documents that we don't false-positive on `%2F` in query strings.

Closes A13-3 (open-redirect on post-login).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/app/src/utils/postLoginRedirect.ts

@@ -1,19 +1,49 @@
 /**
  * Resolve the post-login redirect target. If the caller supplied a `?to=`
- * query that's a same-origin relative path, honour it; otherwise fall back
- * to the auth-store's resolveLandingRoute().
+ * query that is a same-origin, well-formed relative path, honour it;
+ * otherwise fall back to the auth-store's resolveLandingRoute().
  *
- * The `startsWith('/')` + `!startsWith('//')` guard is the **minimum**
- * A13-3 (open-redirect) precaution. Full domain-validation lands in
- * WS-3 PR-B2b.
+ * `isSafeRelativePath` rejects every input that is not a strict relative
+ * path: missing/empty, absolute, protocol-relative (`//`), backslash-bearing
+ * (browsers normalise `\` → `/` in some contexts), control-character-bearing,
+ * or anything the URL constructor parses to a different origin than our
+ * synthetic invalid origin. The URL-constructor check is the authoritative
+ * guard — the prefix and character checks are fast pre-filters.
+ *
+ * Closes A13-3 (open-redirect on post-login). The minimum precaution from
+ * WS-3 PR-B2a (`startsWith('/') && !startsWith('//')`) is now superseded.
  */
+
+const SYNTHETIC_ORIGIN = 'https://__crewli_safe_relative_check__.invalid'
+
+function isSafeRelativePath(to: string): boolean {
+  if (!to || !to.startsWith('/') || to.startsWith('//'))
+    return false
+
+  if (to.includes('\\'))
+    return false
+
+  // eslint-disable-next-line no-control-regex
+  if (/[\x00-\x1F\x7F]/.test(to))
+    return false
+
+  try {
+    const url = new URL(to, SYNTHETIC_ORIGIN)
+    if (url.origin !== SYNTHETIC_ORIGIN)
+      return false
+  }
+  catch {
+    return false
+  }
+
+  return true
+}
+
 export function resolvePostLoginTarget(
   rawTo: string | null | undefined,
   fallback: () => string,
 ): string {
   const to = rawTo ?? ''
-  if (to.startsWith('/') && !to.startsWith('//'))
-    return to
 
-  return fallback()
+  return isSafeRelativePath(to) ? to : fallback()
 }