fix(GUI): rewrite LoadedView options layout at default window size

[ShadyUnderLight]

Problem

At the default window size (640×560), the LoadedView options area had two issues:

1. Content-area collapse: The inner VStack inside the ScrollView had .frame(maxHeight: .infinity), which made SwiftUI assign infinite height to a scrollable child — causing the content to collapse or expand unpredictably.
2. Options clipping: All options (chapter source, output dir, filename template, overwrite policy, output format) were laid out in a single HStack row. With the 260pt chapter-source Picker taking most of the width, the right-side controls ("冲突处理", "输出格式") were compressed or invisible at 640pt.

Solution

1. Remove .frame(maxHeight: .infinity) from the inner VStack so ScrollView sizes content naturally.
2. Add .frame(maxWidth: .infinity) to header and options so they fill the ScrollView's width.
3. Replace the single-row options HStack with a two-row VStack, each row split into labeled sections (caption2 group headers) with fixed min widths — no more invisible controls.
4. Use a dual-Spacer in Row 1 to visually center the output-directory group.

Changes

• GUI/Views/ContentView.swift: Complete rewrite of the optionsSection computed property; structural changes to LoadedView.body