Make project scrollbars visible and interactive

[Zephyr-Song]

What changed

• Changed the four core project fields from conditional overflow-y: auto to an always-present internal vertical scrollbar.
• Added a wider, higher-contrast scrollbar track and thumb for Chromium/WebKit plus Firefox-compatible scrollbar colors.
• Preserved the lower-right vertical resize behavior while distinguishing it from content scrolling.
• Added concise interface guidance explaining mouse-wheel, scrollbar-drag, and resize interactions.
• Added regression coverage for the scrollbar CSS and accessible descriptions.

Why

The previous implementation supported lower-right resizing, but the user specifically needed the right-side internal scrollbar to be visible and draggable for reviewing long text.

Validation

• 52 Python tests passed.
• Playwright + Edge verified all four fields have overflow-y: scroll, stable gutters, and overflow content.
• Mouse-wheel tests moved each field to scrollTop = 320.
• Direct scrollbar-thumb drag tests moved each field to scrollTop > 0.
• git diff --check passed.