fix: drop duplicate manaHref — Credits & Abo is the single billing entry

After merging /mana into the credits workbench app, both manaHref and
creditsHref pointed to /?app=credits, rendering two identical dropdown
entries ("Mana" + "Credits"). Drop manaHref so only the "Credits"
entry remains.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/mana/apps/web/src/routes/(app)/+layout.svelte

@@ -981,7 +981,6 @@
 					currentSyncLabel={syncStatus.label}
 					{appItems}
 					{userEmail}
-					manaHref="/?app=credits"
 					profileHref="/?app=profile"
 					spiralHref="/?app=spiral"
 					creditsHref="/?app=credits"