Improvements

[reversallava63]

This PR fixes the visible flicker on menu navigation and several other
screens, adds a startup splash screen, and moves the UI to a pure-black
dark theme. It also makes two small build/tooling improvements.

Why?

Interactive widgets (menu buttons, bot list rows, time-select cards,
pause overlay items, game-over buttons) were being drawn by pushing
dozens of small eadk_display_push_rect / eadk_display_pull_rect
calls per frame. Because those calls happen at arbitrary phases of the
panel scan, the beam could catch a button mid-update, producing the
characteristic flicker / tearing on arrow-key navigation.

Fix

The core change is to compose each widget into a scratch buffer and
push it to the panel in a single call, gated on eadk_display_wait_for_vblank():

• scratch[200*56] holds the largest widget.
• s_push() waits for vblank, then issues one push_rect.
• Because the transfer lands during retrace and is atomic, the panel
  never sees a partial update.

On top of that:

• list(), overlay(), time_select() and game_over() now only
  redraw the button that lost the highlight and the one that gained it,
  instead of clearing and redrawing every item on each keypress.
• bot_select() was split into bot_row() / bot_info() /
  side_buttons(), so changing bot touches two rows plus the info
  block, and changing side touches three small rects.
• bot_info() pre-clears only the text region instead of the whole
  160×160 right column, so the avatar repaints without a flash.
• rrect_on() blends anti-aliased rounded corners against a known
  background colour rather than reading the screen, which lets the
  widget pre-clears be removed entirely.

Splash screen

splash() at startup fades in a 120×120 image on a black backdrop,
holds, then fades out into the menu. Fade frames are composed into a
framebuf[320*16] band and pushed one band per vblank, so the
transition is smooth and never tears.

Dark theme

• BG is now pure black (0x0000).
• CARD was retuned from a warm grey (0x3A,0x38,0x35) to a cool
  near-black (0x22,0x22,0x25) so buttons read as elevated surfaces
  against black rather than beige tiles.
• Pause overlay and game-over dialog now sit on CARD to match the
  button style, with their own buttons blending corners against CARD.

Testing

• Walked every menu, including arrow-key mashing, on an N0120.
  No flicker.
• Played full games vs. bots and 2-player, checked board, panel,
  clocks, captures, pause overlay, resign, game-over dialog.
• Solved and failed puzzles, tested hint / solution / next.
• Rebuilt from clean and confirmed identical output except for the
  size line.

The splash adds ~28 KB of .rodata (120×120×2). If that's a concern
for the N0110 build, I can downsample to 96×96 or drop the splash
behind a compile-time flag.

[Mason363]

Thank you for this! NumChess has moved to NumPlay, which is where its development continues: https://github.com/Mason363/NumPlay . The flicker fix is being done there, so I'm closing this pull request here.