Address Gadfly review on #6: logout errors, register gating, guard errors

Fixes from the PR #25 adversarial review (graded 23 real / 5 false positive):

Error handling
- onLogout wraps mutateAsync in try/catch: a failed logout no longer
  becomes an unhandled rejection; the user stays put (session still valid)
  and the button offers "Retry sign out" (5 models flagged this).
- Root errorComponent (RouteError): a non-401 /auth/me failure in a
  beforeLoad guard now shows a recoverable "Try again" screen instead of
  blanking.
- Forms drop noValidate, restoring native required/type=email/minLength
  checks before hitting the server.

Correctness
- RegisterPage gates on providers.isPending/isError before rendering, so
  the form no longer briefly appears (submittable) on an SSO-only server.
- TextField id falls back to name then a useId() value, so the label/hint
  associations hold even if a caller omits both.

Maintainability
- Single safeRedirectPath (lib/redirect.ts) replaces the duplicated
  safeRedirect/safeInternalPath (4 models).
- Generic ApiError helpers (apiErrorCode, errorMessage) moved to lib/api.ts;
  LoginPage builds the OIDC URL from the exported API_BASE.
- Divider moved to components/ui/. errorMessage doc clarified.

Verified again in a real browser: register -> /gardens; Sign out -> /login.
tsc --noEmit and vite build clean.

Not changed (graded, with rationale): OIDC deep-link redirect isn't
preserved (needs backend state threading — follow-up); 60s me staleTime in
guards (server is authoritative); the login/register onSubmit catches are
the intended react-query pattern (errors render via mutation state).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

web/src/lib/redirect.ts

@@ -0,0 +1,14 @@
+// Where to land after auth. The router guard stashes the intended destination
+// in ?redirect=, and the login page reads it back; both go through this so a
+// caller-controlled value can never send the user to an external origin.
+
+const DEFAULT_DESTINATION = '/gardens'
+
+/**
+ * Return dest only if it is a same-origin absolute path (starts with a single
+ * '/'); otherwise fall back to the gardens list. Rejects protocol-relative
+ * ('//evil.com') and absolute ('https://…') URLs.
+ */
+export function safeRedirectPath(dest: string | undefined, fallback = DEFAULT_DESTINATION): string {
+  return dest && dest.startsWith('/') && !dest.startsWith('//') ? dest : fallback
+}