feat(client): add mobile layout to settings screen

[NC1107]

Settings is now usable on mobile: section list pushes into detail pages instead of cramming a 320px sidebar onto narrow screens. Desktop two-pane behaviour unchanged.

Improved

• Below the 600 px breakpoint, SettingsScreen now renders a single-pane stack: the section list is the screen, tapping a row pushes into the detail page with a back arrow in the app bar.
• Layout-tier selection now goes through StableLayoutDecision (mirroring HomeScreen), so dragging the window across the 600 px seam no longer tears down the scaffold and drops in-flight detail state.

Behind the scenes

• Split the mobile branch of build into _buildMobileRootPage, _buildMobileHeader, _buildMobileDetailPage, and _handleMobileBack so each function stays well under the S3776 cognitive-complexity budget.
• Added two widget tests at 360x640: one verifies the section list renders without the 320 px desktop sidebar being in the tree; the other verifies tapping a section pushes into the detail page.