Settings > Update never shows an empty status line

[WillTDA]

For the 5 seconds before the launch check, the updater is idle, and the Update tab had no message for idle: the status line was blank.

• Idle now reads "Updates haven't been checked yet."
• The status gains pending, true while the launch check waits. Settings > Update on screen then runs the check at once (the existing update:check, no new channel).
• Any check run before the timer takes its place, so a launch never checks twice. Before, a Check for Updates during the wait was followed by a second check, and the line went back to "Checking for updates…" after it had said "You're up to date."
• buildSettings() tells its caller each tab selected (not the first, while it's still being built).
• docs/updates.md says all this, and lists pending.

Tests: a unit test for the timer skipping after a check, an e2e test for the tab running the pending check once, and the idle copy in the status-line test. 253 unit and 109 e2e pass locally; the update-server specs run in CI's packaged pass.