syncs: improve header label contrast in sync and backup tables

[dragonleopardpig]

Summary

• make sync / backup table header labels reliably readable

Problem
In the sync and backup tabs, table header labels such as "Sync name", "Status",
and "Files" could render with effectively no contrast in this environment.

Fix

• scope the workaround to the sync/backup table headers
• mark the affected headers explicitly
• paint their label text through the proxy style with a readable color

This keeps the fix narrow and avoids a global style override.

Tested on

• NixOS 26.05
• Hyprland
• Qt 5.15