fix(ui): absolute-positioned elements don't reflow on window resize

[ac999]

Problem

When the window is resized larger than the design size (1144×729), only elements using width: parent.width reflow correctly (title text, subtitle, logo, exit button). The absolute-positioned elements stay anchored to their original pixel coordinates and go off the visible area or fail to centre:

• Section 1 card + number badge (x: 279px, y: 262px hardcoded in SectionElement)
• Section 2 card + number badge (x: 279px, y: 437px hardcoded)
• Create Room button (x relative to snd.x + snd.width)
• Join Room button (same)

Root cause

The layout in rust/src/ui.slint uses absolute x/y pixel coordinates throughout SectionElement, CodeRectangle, DataRectangle, and the room action buttons on Screen 0. These were designed for a fixed 1144×729 canvas.

Fix

Convert Screen 0 content from absolute coordinates to Slint layout elements (VerticalLayout / HorizontalLayout / GridLayout), similar to how Screen 2 is already structured. The outer VerticalLayout should drive vertical spacing; the code sections and action buttons should be placed in HorizontalLayouts with alignment: center.

Affected file

rust/src/ui.slint — Screen 0 content block (~line 560–680)