Add dismissable top banner for Nightscout, Dexcom, and heartbeat issues

[bjorkert]

Summary

Failed BG downloads are currently only visible in the log viewer, and a failed Dexcom Share login silently falls back to Nightscout — a user with a bad token, mistyped URL, or wrong Dexcom credentials sees no data with no explanation.

This adds a dismissable banner above the tab view. It is visible on every tab and pushes the content down (up to 3 lines) rather than overlaying it.

What it does

• Nightscout failures: after a failed BG fetch, a rate-limited probe of status.json classifies the cause, so the banner can say why — "The token is invalid.", "A token is required.", "The site was not found.", "A network error occurred." — reusing the same NightscoutError messages the settings screen shows. If the server is reachable but the data fetch still failed, a softer warning says so.
• Dexcom Share failures: ShareError login codes are mapped to friendly text (account not found, incorrect username or password, too many failed attempts). Matching is by substring so both the legacy SSO_Authenticate… codes and the current AccountPasswordInvalid are covered. When Nightscout is configured as backup, the previously silent fallback is now surfaced as a warning ("… Using Nightscout as backup.") while data keeps flowing.
• Frequent heartbeat dropouts: when the Bluetooth heartbeat becomes regularly delayed — 5 or more beats arriving later than 1.5× the expected interval within the last hour, a typical symptom of a dying transmitter battery — a warning suggests checking the transmitter/RileyLink/pod battery or Bluetooth range. A single late beat never triggers it; gaps large enough to indicate out-of-range, Bluetooth off, or a suspended app reset the count instead of adding to it. The warning clears after a clean hour, or immediately on device disconnect/switch.
• Clearing: a banner disappears automatically when its source recovers (next successful fetch, clean heartbeat window) or when that source is deconfigured (URL/credentials removed, device disconnected).
• Dismissal: tapping ✕ hides the banner. The same error stays hidden for 30 minutes (then re-appears if still occurring); a different error shows immediately. Repeated failing retries reuse the same message, so the banner doesn't re-animate every 10–60 s cycle.
• Severity: when multiple sources have problems, the more severe message wins (error > warning > info); dismissing it reveals the next one.

The banner system is generic (BannerManager.report/clear with a custom source), so other important notices can use it later.

Also fixed

NightscoutUtils.verifyURLAndToken classified transport errors backwards: DNS/connection failures were reported as siteNotFound and everything else as networkError. It now distinguishes them by URLError code, which also improves the status label in Nightscout settings.

[bjorkert]