Make Settings scrollable and fix sync-mode radio semantics

[in-jun]

Two accessibility fixes for the Settings screen.

• Wrap the settings content Column in verticalScroll(rememberScrollState()) so that at large font scales or in landscape the lower controls (Wi-Fi-only switch, mass-deletion guard) stay reachable instead of being pushed off-screen, matching the pattern already used in SetupScreen.
• Give the sync-mode rows role = Role.RadioButton on .selectable and drop the inner RadioButton's own onClick (set to null) so each option is announced as a single radio option with one focus stop, instead of two separately focusable targets.

Fixes #48
Fixes #101

[in-jun]

Both changes look correct and match the analysis in #48 and #101.

Scrollability (#48): Wrapping the content Column in verticalScroll(rememberScrollState()) fixes the off-screen-controls problem at large font scales / landscape. Worth noting the modifier ordering differs slightly from SetupScreen: here the .padding(16.dp) sits inside the scroll region while the Scaffold inset .padding(padding) stays outside it. That's actually the better arrangement — the nav-bar inset stays fixed and non-scrollable, and the 16.dp gives the last control trailing space when fully scrolled. No layout regression.

Radio semantics (#101): role = Role.RadioButton on .selectable plus onClick = null on the inner RadioButton is the canonical Compose pattern — the button becomes purely visual and its semantics merge into the single selectable row, so TalkBack announces one correctly-labeled radio option with a single focus stop instead of two targets.

Imports are all used, ModeOption is private with no other call sites, and the change is minimal and self-contained. LGTM.