[Accessibility] Add aria-live regions for status changes

[accius]

Sub-issue of #997 (accessibility umbrella).

Problem

Screen-reader users miss status changes that sighted users notice visually — band changes in WSJT-X, new DX spots arriving, satellite pass beginning/ending, lightning proximity alerts, WSJT-X relay connect/disconnect, Cloud Relay state changes, etc. Without aria-live regions, these updates pass silently.

What we want

Identify the meaningful status-change events and wrap their announcements in appropriately-prioritised aria-live regions:

• aria-live="polite" — non-urgent: new spots arrived, satellite pass in 10 min, weather refresh
• aria-live="assertive" — urgent: lightning within proximity radius, severe weather alert, relay disconnected mid-session
• role="status" / role="alert" — semantic roles where they fit better

Be careful with frequency: a live region that announces every single new DX spot would be unbearable. Better to debounce ("3 new spots in the last minute, ZL2ABC the newest") or only announce notable transitions.

Scope

A short audit of status indicators in OHC, paired with a small set of debounced live-region wrappers in the appropriate components. Probably touches: WSJT-X integration, DX Cluster panel, Satellite predictions, Lightning layer, Weather alerts.

Tester wanted

If you use a screen reader, your feedback on cadence (too chatty vs. too quiet) would be invaluable.

[accius]

First pass shipped in 79fc1a4 on Staging. Covers three of the targets in the issue body:

• Weather alerts (NWS) in WeatherPanel — each alert div now has role="alert". New severe-weather alerts announce assertively as they appear.
• Lightning proximity — the "X strikes detected" wrapper inside the proximity panel template now has role="alert" aria-live="assertive". Announces on the 0 → N transition. May re-announce when the strike count or closest-distance changes within an active warning, which is acceptable for a life-safety indicator. Smarter transition-only announcement is a follow-up if it proves chatty in practice.
• WSJT-X relay status — the "Waiting for WSJT-X" empty state in PSKReporterPanel now has role="status" aria-live="polite" on its container. Transitions between disabled / disconnected / connected announce politely. Container disappears once decodes start flowing, so it doesn't keep announcing.

Still open (the part where cadence input from a screen-reader user is the bottleneck):

• Debounced DX-spot arrival summary ("3 new spots in last 60s, ZL2ABC the newest")
• Satellite-pass start / end announcements
• Band-change narration when the active WSJT-X mode shifts
• Cloud Relay state transitions

Leaving this issue open as the tracker for the second pass. If you use a screen reader and try out the three above, feedback on whether the cadence feels right would be invaluable, especially on the lightning re-announcement frequency during active storms.