remove fake brightness feature

[zxkmm]

TLDR:

• we have a weird lagging issue years ago, caused by fake brightness.
• it got "fixed" later by assuming the slow part seems from repetedly read from p.mem.
• the fixes worked for years.
• it's broken again now and the assume fix is probably wrong too.
• we have to remove it cuz in current SPI flash XIP structure it is too expensive to fix.

[gullradriel]

Ok, that's your call bro.

[Copilot]

Pull request overview

Removes the performance-heavy fake brightness feature while preserving persistent-memory layout compatibility.

Changes:

• Removes fake-brightness rendering and cached state.
• Removes related settings and status-bar controls.
• Retires persistent fields as reserved entries.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
firmware/common/portapack_persistent_memory.hpp	Removes fake-brightness APIs.
firmware/common/portapack_persistent_memory.cpp	Retires stored fields while preserving layout.
firmware/common/portapack_io.hpp	Removes per-pixel brightness transforms.
firmware/common/portapack_io.cpp	Removes brightness cache accessors.
firmware/common/lcd_ili9341.cpp	Removes brightness cache refresh.
firmware/application/ui_navigation.hpp	Removes the status-bar brightness button.
firmware/application/ui_navigation.cpp	Removes brightness status behavior.
firmware/application/external/debug_pmem/ui_debug_pmem.cpp	Removes obsolete debug fields.
firmware/application/apps/ui_settings.hpp	Removes brightness settings controls.
firmware/application/apps/ui_settings.cpp	Removes brightness settings logic.

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