Fix dark footer text contrast in RCB and signal dialogs

[masarray]

Field issue

Two dark navy dialog footers were rendering their summary/status captions with dark foreground colors, making the text nearly unreadable:

• RCB Export Filter — Legacy SAS
• IEC 61850 Signal Selection

Fix

• RCB export footer primary summary: bright off-white #F4F8FC.
• RCB removal/retention summary: bright light blue #93C5FD.
• Signal-selection footer primary selection count: bright off-white #F4F8FC.
• Signal-selection visible-count secondary caption: light steel #C9D8E5.
• Footer separator: brighter muted steel #8FA8BC.
• Scope is limited to the dark footer captions; table/grid text and button styling are unchanged.

Regression coverage

Adds source regression guards with named footer elements so future theme/layout changes cannot silently restore dark-on-dark footer text.

Final branch contains only the two XAML changes and the regression test; temporary patch tooling was removed before PR creation.

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