Menu bar: Cursor Keychain reader tries a second account after a Deny #1928

Found during the review of #1926.

mac/Sources/CodeBurnMenubar/Data/CursorSafeStorage.swift reads the Safe Storage password from the Keychain by trying the accounts Cursor Key and then Cursor (accounts at line 12, readPassword() at lines 106-108). Any failure on the first account falls through to the second, a user Deny included. So a user who clicks Deny on the Cursor Key prompt can get a second prompt for Cursor straight after.

The Keychain consent policy (#1802) says stop at Deny. The reader should distinguish "item not found" (try the next account) from "access denied or cancelled" (stop, report locked), and the shared unlock from #1926 should record the Deny so Cursor and Grok Bot both stay quiet until the next user action.

Affected: macOS menu bar, Cursor and Grok Bot quota, only on Keychain prompt Deny.