*: Add settings page with theme, text size, avatar shape, and content options
New SettingsView with 6 persisted settings: theme mode (system/light/dark),
dynamic color toggle, post text size (S/M/L), avatar shape (circle/rounded),
reply filtering (none/normal/strict), and show labels toggle. Settings are
backed by DataStore and apply immediately across all views.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>