refactor(story): use shared scrollbar module in story overlay

[forketyfork]

The story overlay was the last holdout with its own scrollbar drawing code. FullscreenOverlay.renderScrollbar() did the job with plain SDL FillRect calls — flat rectangles, fixed alpha, no fade, no rounding, no hover state. Every other overlay (reader, diff, terminal) already used scrollbar.zig, which gives you a rounded gradient thumb, fade-in/out animations, and full mouse interaction.

This removes renderScrollbar() and brings the story overlay in line with the rest. story_overlay.zig now holds a scrollbar.State field and calls scrollbar.computeLayout() + scrollbar.render(). Mouse events are wired: wheel triggers activity, left-click on the thumb starts a drag, left-click on the track jumps the position, motion handles drag updates and hover, and button-up ends the drag. computeWrapCols now uses scrollbar.reservedWidth() instead of a hardcoded 10px estimate.

Closes #226

Test plan

• Open a story file with enough content to scroll (e.g. architect story <some-long-story.md>)
• Scroll with the mouse wheel — the scrollbar should fade in, then fade out after ~1.5s of inactivity
• Hover over the scrollbar — it should stay visible and the thumb should brighten
• Drag the thumb — the content should follow
• Click on the track above or below the thumb — the view should jump to that position
• Compare the scrollbar appearance to the reader overlay (Cmd+R) — rounded thumb, same visual style

[Copilot]

Pull request overview

Refactors the story overlay to use the shared scrollbar.zig component (matching reader/diff/terminal overlays) and removes the bespoke SDL FillRect scrollbar implementation.

Changes:

• Add scrollbar.State to StoryOverlayComponent and render the shared scrollbar via scrollbar.computeLayout() + scrollbar.render().
• Wire mouse interactions for wheel activity, thumb dragging, track-click jumps, and hover-driven fade behavior.
• Update wrap column computation to reserve width using scrollbar.reservedWidth() instead of a hardcoded estimate.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/ui/components/story_overlay.zig	Integrates shared scrollbar state/rendering and hooks up mouse events + wrap-width reservation.
src/ui/components/fullscreen_overlay.zig	Removes the old renderScrollbar() implementation now superseded by scrollbar.zig.

Comments suppressed due to low confidence (1)

src/ui/components/story_overlay.zig:472

• The cursor state transition logic compares want_pointer against was_pointer derived only from the previous anchor/link hover state. Since want_pointer now includes scrollbar hover/drag, moving off the scrollbar can leave the cursor stuck as a pointer (because was_pointer doesn't include prior scrollbar hit/drag state). Track previous scrollbar-hit/hover/drag state as part of was_pointer, or set the cursor unconditionally based on want_pointer each motion event.

                const want_pointer = self.hovered_anchor != null or self.hovered_link != null or self.scrollbar_state.dragging or scroll_hit != .none;
                const was_pointer = prev_hovered_anchor != null or prev_link != null;
                if (want_pointer != was_pointer) {
                    const cursor = if (want_pointer) self.pointer_cursor else self.arrow_cursor;
                    if (cursor) |cur| _ = c.SDL_SetCursor(cur);

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