Font size slider in Settings

What's new

• Font size slider. A new slider in Settings (under Display) lets you scale all text from 80% to 140% of the default size. The preference persists across launches. Useful if the default text is too small to read comfortably, or if you want a more compact view.

• Settings layout tidied up. Sections are reordered (Behavior, Display, Notifications), with more breathing room between groups and slightly larger section labels.

Internal

• Custom fontScale SwiftUI environment key with a ScaledFontModifier that computes actual point sizes per text style. Every .font(.caption) etc. in the popover is now .scaledFont(.caption), which reads the environment and renders at the correct size. No bitmap scaling, so text stays crisp at all sizes.
• AppSettings.fontSize is clamped to 0.8...1.4 on both load and write. A single fontSizeRange constant is shared by the settings slider and the clamp logic.
• Percentage label uses .rounded() to avoid floating-point display glitches (e.g. 109% instead of 110%).