Settings: add explanatory footers and split social keyboard, streaming and timeline fetch into sections

[Dimillian]

Motivation

• Improve clarity of the main settings screen by adding short explanatory footers for a few toggles.
• Surface the isSocialKeyboardEnabled, streamHomeTimeline, and fullTimelineFetch options as their own sections so their meaning is clearer.
• Keep unrelated toggles grouped under the existing "Other" section.

Description

• Added three new sections implemented as socialKeyboardSection, streamHomeTimelineSection, and timelineFetchSection in SettingsTab.swift and inserted them into the Form above otherSections.
• Removed the isSocialKeyboardEnabled, streamHomeTimeline, and fullTimelineFetch toggles from the existing otherSections and reintroduced them inside their dedicated sections.
• Each new section includes a footer Text explaining what the toggle does and preserves the existing look via .listRowBackground(theme.primaryBackgroundColor) where applicable.
• Kept remaining toggles unchanged and preserved the otherSections header and behavior.

Testing

• No automated tests were run for this change.

---

Codex Task