ui_credits: merge, rewrite and fix scrolling credits screen

Merged ui_rally_credits.c (static) and ui_credits.c (scrolling) into a
single ui_credits.c. The static version is replaced entirely by the
scrolling implementation.

Credits data cleaned up: all UI_PULSE flags removed, iD Software section
condensed from 18 lines with individual role headers to 7 lines under a
single "Based on Quake III Arena" heading. Q3Rally section headers now use
orange accent colours to match the rest of the UI.

Fixed a loop break bug that caused a black screen once the first line
scrolled off the top - the loop now runs to completion on every frame.
End-of-credits detection is based on the calculated total height instead
of a hardcoded pixel threshold.

Edge fade reworked: alpha is sampled at line centre rather than top edge,
and negative screen positions are explicitly clamped, fixing a visual
glitch where lines briefly reappeared after fading out at the top.

Background shader is now always drawn, including after all text has
scrolled off, preventing a black flash at the end. A configurable dim
overlay (BACKGROUND_DIM) reduces shader brightness without replacing it.
color_background is conditionally compiled out when BACKGROUND_SHADER is
active to silence the unused-variable warning.

ui_rally_gfxloading: framerate-independent smoothing and cleanup

Replaced the framerate-dependent smoothing factor (0.08f per frame) with
a delta-time based lerp using SMOOTH_LERP_SPEED (4.5/s), giving consistent
animation speed regardless of framerate.

Introduced NUM_STAGES define to replace ARRAY_LEN(stages) at all three
call sites (ExecuteStage, UpdateProgress, MenuDraw), making the stage
count explicit and easier to maintain.

Car icon draw position raised by 10px.

Updated the ladder service

Author: Perle2212