fix: preserve cached usage when deleting accounts

[Bl0ck154]

Deleting an account currently calls loadAccounts() without preserving usage, which clears cached usage for every remaining account and makes all cards temporarily fall back to Fetching usage....\n\nThis changes the delete flow so that:\n- deleting an inactive account removes only that account from local state and leaves every other account's cached usage untouched;\n- deleting the active account still reloads account metadata because the backend may promote another account to active, but does so with preserveUsage=true;\n- no global usage refetch is triggered by account deletion.\n\nValidation:\n- frontend production build passes (tsc && vite build).\n- branch is one commit directly on top of current upstream main; only src/hooks/useAccounts.ts is changed.