feat(help-dialog): add scroll direction arrow indicator

[dungxnd]

Why?

It look weird to have no scrollbar and every dialog just have the same height, so adding an arrow just to improve UX that they have more on the dialog.

Changes

In EffectSection, the help dialog's text block now tracks scroll position and overlays a boundary-aware arrow hint:

• ↓ at the bottom — appears when the dialog first opens and there is content below (scroll position is at the very top). Tells the user they can scroll down.
• ↑ at the top — appears once the user has scrolled all the way to the end. Tells them they can scroll back up.
• Neither arrow is shown during active scrolling — only at the two resting boundaries.
• Both arrows fade in and out via AnimatedVisibility with fadeIn/fadeOut.
• The arrow strip uses a solid surfaceContainerHigh background — the same color token M3 AlertDialog uses for its container — so it blends naturally without any visible seam.
• No dedicated layout space is reserved; the arrows are overlaid on the scroll area via a Box.

[dungxnd]

[likelikeslike]

The capped dialog height is a reasonable design choice to me. Without it, help sections with long descriptions would stretch the dialog to fill most of the screen, limit it to half the window height keeps the dialog compact and consistent.

[dungxnd]

IKR, but I'm not talking about height, I'm talking about UX, when sometime description for long one is the same with the one that fit right at that capped height -> kinda confusing about the scrolling, so add an arrow indicator would be nice.