Enable two-axis scrolling for dashboard widgets

[multiplex55]

Motivation

• Allow widget content to overflow horizontally as well as vertically so wide widget bodies can be scrolled instead of clipped.
• Preserve the previous vertical height constraint behavior by keeping the scroll area height limited to body_height.
• Ensure slot clipping still enforces slot bounds while permitting horizontal scrolling inside the slot via the existing clip-rect intersection.
• Add tests to validate both-axis scrolling behavior and record scroll viewport information for assertions.

Description

• Replaced egui::ScrollArea::vertical() with egui::ScrollArea::both() and retained .auto_shrink([false; 2]) and .max_height(body_height) so vertical height is constrained while horizontal overflow is allowed.
• Kept ui.set_clip_rect(ui.clip_rect().intersect(slot_clip)) to ensure slot bounds are respected while allowing horizontal scrolling within the slot.
• Added test instrumentation SCROLL_VIEWPORTS and OVERFLOW_RECORDS, an OverflowWidget test widget, overflow_registry, and helper functions take_scroll_viewports() / take_overflow_records() to capture viewport and allocated content rectangles.
• Added a unit test scroll_area_allows_horizontal_and_vertical_overflow which renders an oversized widget and asserts that the painted/allocated rect is larger than the scroll viewport and that scrollbars are exposed as expected.

Testing

• Added the unit test scroll_area_allows_horizontal_and_vertical_overflow to cover two-axis overflow behavior.
• Existing unit tests were left intact and continue to record scroll visibility via SCROLL_VISIBILITY_RECORDS.
• No automated test run was performed as part of this change (cargo test was not executed in this rollout).
• Manual code compilation / CI is recommended to verify the new test and existing test-suite pass.

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Codex Task