Add LOG_WARN on all silent error returns in lib_nbgl

98 cases across 11 source files where a function returned silently on
an error condition (NULL pointer, invalid parameter, unexpected object
type, exhausted pool) with no diagnostic trace. Each now emits a
LOG_WARN with the function name and the failing condition.

Most critical cases:
- nbgl_layout.c: ~64 silent returns in AddHeader/AddFooter/AddUpFooter
  on layoutAddCallbackObj failure — invisible in production since
  LOG_FATAL is a no-op outside debug builds
- nbgl_layout_keypad.c / nbgl_layout_keyboard.c: type-check returns on
  corrupted internal state (wrong obj.type) with no indication
- nbgl_use_case.c: public API entry points (keypad, keyboard, choice)
  silently ignoring out-of-range or NULL parameters
- nbgl_layout_nanos.c: two TODOs ("warning for screenshots") converted
  to actual LOG_WARN calls
- nbgl_step.c / nbgl_flow.c: context-lookup failures producing silent
  no-ops at the use-case orchestration level

No logic changed; only LOG_WARN calls added immediately before each
concerned return.