fix(gui): pack XVTR setup fields with trailing stretch (#3454)

[jensenpat]

Summary

The XVTR tab in the Radio Setup dialog renders with large, uneven vertical gaps between every field row (Name, RF Freq/IF Freq, LO Freq/LO Error, RX Gain/RX Only, Max Power), unlike the tightly-packed sibling tabs (GPS, Audio, TX, RX, Antennas, Filters, APD, USB Cables, Peripherals).

Fixes #3454.

Root cause

RadioSetupDialog::buildXvtrTab() was the only Radio Setup tab whose outer QVBoxLayout did not terminate with addStretch(1). Every sibling builder does:

Tab	trailing vbox->addStretch(1)
TX (buildTxTab)	✅
RX (buildRxTab)	✅
Audio (buildAudioTab)	✅
Filters (buildFiltersTab)	✅
APD (buildApdTab)	✅
XVTR (buildXvtrTab)	❌ (before this PR)

Without that trailing stretch, the QVBoxLayout had no flexible element to absorb leftover vertical space, so it handed all of it to the QTabWidget. The tab widget grew to fill the dialog, and the inner per-transverter QGridLayout — which has 5 content rows and no row-stretch factors — distributed the surplus height evenly across the rows. That even distribution is exactly the excessive inter-row spacing reported in the issue.

Fix

Add a single trailing stretch after the tab widget:

vbox->addWidget(xvtrTabs);
vbox->addStretch(1);   // <-- added
return page;

The stretch now absorbs the leftover vertical space, so the QTabWidget collapses to its compact size hint and the inner grid receives only its natural height — packing the field rows tightly at the top and matching the layout density of the other tabs. Behaviour on dialog resize (content stays top-aligned, empty space below) is now identical to the sibling tabs.

One-line, layout-only change; no behavioural or data-path impact.

Testing

• Built locally (cmake --build build -j22, RelWithDebInfo, macOS) — clean, [1042/1042].
• Deployed to a FLEX test bench (FLEX-8600) and confirmed in Radio Setup → XVTR: field rows are now compact and the vertical rhythm matches the Audio/TX tabs. No regression in the per-transverter sub-tabs or the "+" add-transverter tab.

💻 Generated with Claude Code (Opus 4.8) with architecture by @jensenpat