Remove dead _previous_frequency state from BigFrequency widget

[Copilot]

BigFrequency retained a _previous_frequency member that was no longer used in paint(), leaving dead state in the class and making the header disagree with the implementation.

Brief description of what you did

• Removed rf::Frequency _previous_frequency{~0LL} from BigFrequency in ui_widget.hpp — it was dead state after the paint guard was removed.
• Removed the if (_frequency != _previous_frequency) guard from paint() — paint() is now unconditional, which is correct since the widget framework gates it.
• Moved the dirty-check to set() to preserve the optimization (skip repaints when frequency hasn't changed) without needing the extra member:

void BigFrequency::set(const rf::Frequency frequency) {
    if (_frequency != frequency) {
        _frequency = frequency;
        set_dirty();
    }
}

This keeps the redraw behavior correct while eliminating the stale member.

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Proof that your changes work

Code-only cleanup; no behavioral change to observable output.

🖥️ Proof it compiles

N/A — trusted contributor

📱 Proof of testing on a real device

N/A — no functional change; only removes dead state and restructures the dirty-check.

📡 Proof against a real emitter/receiver (if applicable)

N/A — no RF changes.

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📚 Wiki documentation commitment

• I will (or already) create(d) wiki document for my newly added feature

N/A — internal widget refactor, no user-visible feature change.

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Checklist

• Kept changes minimal and limited to necessary files
• Verified functionality remains intact and code compiles
• Attached proof that the code compiles successfully (or marked N/A as a trusted contributor)
• Attached proof of testing on real PortaPack hardware (or marked N/A as a trusted contributor)
• Attached proof of testing against a real emitter/receiver (if RF-related), or marked N/A with justification
• I understand that by getting this PR merged, I am implicitly agreeing to create or update the corresponding wiki page (including a main-screen screenshot, description, controls, and limitations)
• I own all rights to this code (i.e., all code contained in this PR), including compliant usage rights for third-party libraries, and I agree that this code is licensed under the license of this project (GPL-3.0).
• If any third-party libraries are used, I confirm that their licenses comply with the requirements for contributing to this repository.
• Reviewed the Contributing Guidelines

[gullradriel]

Looks good to me

[zxkmm]

Ok