fix(web): render settings sections as real tabs

[gbasin]

Switching tabs in the settings view changed the URL but not the content. It turned out to be broken in two ways.

The sidebar looked like tabs but was implemented as scroll anchors — every section rendered at once, and clicking one only called scrollIntoView.

1. The scroll container was never height-constrained

SettingsSurface.tsx:111 was a plain block div, so its child's flex-1 did nothing and the scroller sized to its content:

min-h-0 flex-1                    clientH 327   ← wrapper, correctly constrained
  min-h-0 flex-1 overflow-y-auto  clientH 682   ← scroller, NOT constrained
                                  scrollH 682   ← scrollHeight === clientHeight

scrollHeight === clientHeight makes scrollIntoView a guaranteed no-op. The overflow escaped up to the shell's overflow-hidden (scrollH 971 vs clientH 420) and was clipped — at a 420px-tall window, Agents and About sat at y=621 and were unreachable by any means, with no scrollbar to signal it.

2. Even scrolling correctly, it read as dead

The whole surface is ~682px tall, so at normal desktop heights everything already fit and there was nothing to scroll to. With no section headings in the content column either, clicking produced zero feedback.

Fix

Render only the active section. That removes the scroll dependency entirely, so section switching cannot silently no-op at any viewport size. The nav becomes a fixed rail with only the panel scrolling, and the active section gets a heading that labels the panel via aria-labelledby.

This also deletes the now-dead sectionRefs / useEffect / SettingsSectionDivider machinery — net −57 lines in the component.

The URL grammar is unchanged: /settings/:section still deep-links and restores, and Back/Forward still work.

Verification

Driven in a real browser at three viewports. Each tab swaps its rows:

URL	heading	rows rendered
/settings	Appearance	Theme, Accent, Text size, High contrast, Motion
/settings/notifications	Notifications	Device, Messages, Agents, Calls
/settings/connections	Connections	GitHub
/settings/agents	Agents	Claude Code, Codex
/settings/about	About	—

At 380px tall the panel now genuinely scrolls (clientH 287 / scrollH 290) with the last row reachable. Mobile keeps its horizontal pill strip.

• pnpm lint clean, tsc --noEmit clean
• 857 unit tests pass
• 120/120 e2e pass

Tests

• New guard in SettingsSurface.test.tsx asserting inactive sections are absent from the DOM — this is what would have caught the original bug.
• notificationSettings.test.tsx asserted across sections, so it now opens the section holding the controls under test.