Customize top-level sidebar items (#4203)

* feat(app): customize top-level sidebar items

Let users reorder and hide built-in and plugin sidebar rows from Appearance settings while keeping the preference device-local. Add Search as a first-class row and standardize section explanations as header info tooltips.

* fix(app): preserve consecutive sidebar preference edits

Derive each sidebar change from the latest cached settings and retain unavailable plugin slots while editing the visible list. Keep E2E interaction selectors tied to accessible control names.

* test(app): isolate provider settings unit boundary

Keep the legacy provider-section test focused on provider behavior now that its shared section shell can render the settings info control.

docs/design.md

@@ -120,6 +120,8 @@ Workspace and chat surfaces use the full width — these are working surfaces, n
 
 Sections sit apart. `<SettingsSection>` owns its own bottom margin; the next thing is wrapped in another `<SettingsSection>`. The agent-list `sectionHeading` carries the same `marginTop`/`marginBottom` rhythm (`packages/app/src/components/agent-list.tsx:511-517`). Adding `marginBottom` to a section is wrong.
 
+A section or group explains itself through the `info` prop on `<SettingsSection>` or `<SettingsGroup>` — an info icon beside the header that opens a tooltip (`packages/app/src/screens/settings/settings-info-tip.tsx`). A muted paragraph between the header and the card is wrong: it sits in the section's own gap, so it reads as a second heading rather than as prose belonging to the header. Explanatory copy that describes one row belongs to that row, as `settingsStyles.rowHint` inside the card.
+
 Cards inside a section sit closer than sections. Rows inside a card touch — only the divider separates them. The rhythm is page → spacious; section → spacious; card → tight.
 
 Rows have generous vertical padding: roughly 16px of content plus 16px of vertical padding for settings rows, 8–12px for sidebar list items where many rows must fit. Compressing rows below the established density to fit more on the screen is wrong. Too many rows means more cards or more sections, not smaller rows.
@@ -248,6 +250,7 @@ New status pills use `<StatusBadge>`. Identity, shortcut, and interactive link b
 - `fontWeight.medium` on row titles, body text, button labels, badge text, or `<SidebarCallout>` titles. Medium is reserved for the structural-label tier described in §3 — section labels, modal/sheet titles, dense metadata emphasis, and tight action labels. Anything else is `normal`. `<ScreenTitle>` is responsive `400/300` and is never overridden.
 - `<Pressable>` wrapping `<Text>` to make a button. `<Button>` exists.
 - Bare `<Text>` for a section header inside settings. `<SettingsSection>` exists.
+- A muted paragraph between a section header and its card. Section-level explanation is the header's `info` tooltip (§7).
 - A "Settings" CTA on a detail page. Detail pages are settings; settings is reached from the sidebar, the host entry, or a row's kebab menu.
 - The word "checkout" in UI strings or identifiers. The term is "workspace".
 - New color tokens or hardcoded hex outside the palette. The identity color table is the documented exception (§13), not a license.