Fix any type usage in the saas/ folder (#5934)

# Description of Changes
Ages ago I made #4835 to try and fix all the `any` type usage in the
system but never got it finished, and there were just too many to review
and ensure it still worked. There's even more now.

My new tactic is to fix folder by folder. This fixes the `any` typing in
the `saas/` folder, and also enables `no-unnecessary-type-assertion`,
which really helps reduce pointless `as` casts that AI generates when
the type is already known. I hope to expand both of these to the rest of
the folders soon, but one folder is better than none.

frontend/src/saas/routes/Signup.tsx

@@ -62,12 +62,9 @@ export default function Signup() {
       setError(null)
 
       console.log('[Signup] Initiating anonymous sign-in...')
-      const { data, error } = await signInAnonymously()
+      const { data } = await signInAnonymously()
 
-      if (error) {
-        console.error('[Signup] Anonymous sign-in error:', error)
-        setError(`Failed to create guest account: ${(error as any)?.message || 'Unknown error'}`)
-      } else if (data.user) {
+      if (data.user) {
         console.log('[Signup] Anonymous sign-in successful, refreshing session...')
 
         // Refresh session to ensure backend endpoints are properly synchronized