fix: re-introduce debounce logic in MomentaryButton for NewUI

[The-Bober]

Problem

The de-bounce logic present in the original UI was not carried over
during the NewUI / MomentaryButton refactor. Without it, a single
physical button press can generate multiple edges within a few
milliseconds, causing spurious clicks and unreliable multi-click
detection.

Fix

• Added DEBOUNCE_MS (50ms) constant to MomentaryButton.cpp
• Added _last_edge_ms member to track the last valid edge timestamp
• Inside check(), any btn != prev transition that occurs within
  50ms of the previous valid edge is silently ignored via a goto
• All existing logic (long press, multi-click window, repeat click)
  is unchanged

Behaviour

• Debounce fires first, before the 280ms multi-click window
• Only clean, intentional edges reach the click-counting logic
• Matches the intent of the original OrigUI implementation

Fixes #2539