[Accessibility] Screen-reader spot list announcements + semantic table markup

[accius]

Sub-issue of #997 (accessibility umbrella).

Problem

The DX Cluster, POTA, SOTA, WWFF, and PSK Reporter spot lists are built from <div> grids today. Screen readers can read them, but row/column relationships aren't communicated — a user can't ask the reader to "next column" or "this row's frequency" the way they could with a real <table>.

Additionally, frequencies are rendered as plain numbers (e.g. "14.074") which most screen readers will pronounce as "fourteen point zero seven four" — usable, but not the natural ham announcement "fourteen point zero seven four megahertz" that gives context immediately.

What we want

1. Convert the spot grids to semantic tables (<table>, <thead>, <tbody>, <tr>, <th>, <td>) with scope attributes on headers. Keep the same CSS grid look — just under semantic markup.
2. Add a hidden <span class="visually-hidden"> after each frequency that says "megahertz" so screen readers get the unit, while sighted users see just the number.
3. Same for age ("3 m ago" → screen reader hears "3 minutes ago" via aria-label or visually-hidden span).

Scope

Affects DXClusterPanel, ActivatePanel (covers POTA/SOTA/WWFF), and PSKReporterPanel. Be careful not to regress the visual density work done in #995.

[accius]

Done in 33ed861 on Staging.

All three items in the issue body addressed:

1. Semantic table markup: applied as ARIA roles on the existing div grids (role="table" / role="row" / role="columnheader" / role="cell") rather than converting to real <table> elements. Real <table> plus display: grid would have lost the table role in the accessibility tree, so the ARIA-on-divs path preserves the [FEATURE] Toggle visibility of spotter info in DX Cluster panel #995 density work pixel-for-pixel while still giving screen readers a proper table to navigate. Hidden header rows added on all three panels so column navigation has labels even though the visual UI omits them.

2. Frequency "megahertz" announcement: new .visually-hidden utility class in main.css (standard clip-path pattern), and a hidden <span> megahertz</span> after each numeric frequency in DXClusterPanel, ActivatePanel, and PSKReporterPanel.

3. Age unit expansion: handled via aria-label on the time/age cells. DXClusterPanel gets a new formatSpotTimeAriaLabel() that turns "5m 14:30z" into "5 minutes ago, 14:30 UTC". PSKReporterPanel gets a formatAgeAriaLabel() that expands "5m" / "2h" into "5 minutes ago" / "2 hours ago", and the combined mode/SNR/age cell announces as "FT8, SNR plus 3 decibels, 2 minutes ago". ActivatePanel's time cell was left as-is because spot.time format varies by source (POTA / SOTA / WWFF all hand back different shapes), so a reliable aria-label needs per-source parsing — open follow-up if it matters.

Bonus: the mutual-reception ★ in PSKReporterPanel now has aria-label="Mutual reception, QSO possible" so screen readers don't just hear "star".

Localization caveat: the new aria-label strings and " megahertz" / "minutes ago" expansions are hard-coded English. The visible UI text still goes through t() and stays localized. Putting the screen-reader strings through i18n would meaningfully grow the translation burden across all language files, and the issue body didn't require it, so it's deferred. If a non-English screen-reader tester reports the labels being read as English in their locale, we can reopen and route through t().

Closing the issue. Tester feedback still wanted on cadence and announcement quality, will be tracked under umbrella #997.