refactor(settings): merge Sources into Thresholds as "Targets & Sources"

The two former settings sections (`/settings/thresholds` and
`/settings/sources`) covered the same conceptual shelf — per-metric
configuration. Reading them together matches how users actually
think about each metric ("for this metric, which sources do I trust
AND what range counts as healthy?") and collapses two adjacent
sidebar slots into one.

`<ThresholdsSection>` now hosts both editors stacked under one page
header: the per-metric threshold ranges on top, the per-metric source
priority + two-axis device-type ladder below. The two mutation flows
(`/api/user/thresholds` and `/api/auth/me/source-priority`) stay
distinct so a save on one half never disturbs the other.

`<SourcesSection>` gained a `mode` prop. The default `"standalone"`
behaviour is preserved for any caller that still mounts it directly;
the new `"embedded"` mode drops the inner header so the assistive-tech
reading order stays one combined surface, not two.

The `sources` slug was dropped from `SETTINGS_SECTION_SLUGS` (down to
ten sections), and the Sources entry was removed from the in-shell
sidebar + mobile chip-strip. `/settings/sources` stays alive as a
`permanentRedirect` to `/settings/thresholds`, so external bookmarks
(iOS Settings deep-links, docs, anything that linked the URL) follow
through unchanged.

Tests updated: `SETTINGS_SECTION_SLUGS` size assertion drops from
eleven to ten; `<ThresholdsSection>` mount test now expects both
editor mount-points; the section title moves from "Personal
thresholds" / "Persönliche Zielwerte" to "Targets & Sources" /
"Zielwerte & Quellen".

Per `.planning/research/v1434-r-2-carryover-scope.md` §7 and
`.planning/round-v1433-audit-menu.md` §7.1 (item 3).

src/components/settings/settings-shell.tsx

@@ -21,7 +21,6 @@ import {
   Bell,
   Download,
   KeyRound,
-  Layers,
   LayoutDashboard,
   Link2,
   Settings2,
@@ -73,6 +72,15 @@ interface SettingsSection {
  * keeps emitting the page — but it no longer appears in the in-page
  * navigation. Direct links (the user-card dropdown, the public
  * `/about` permalink) continue to resolve to the same component.
+ *
+ * v1.4.34 IW-D — `sources` was merged into `thresholds` (now titled
+ * "Targets & Sources" / "Zielwerte & Quellen"). The combined section
+ * surfaces per-metric source priority alongside the threshold inputs
+ * on a single page. `/settings/sources` stays alive as a
+ * `permanentRedirect` so external bookmarks keep working; the slug
+ * was dropped from `SETTINGS_SECTION_SLUGS` because the dynamic
+ * `[section]` route no longer serves it. Per
+ * `.planning/research/v1434-r-2-carryover-scope.md` §7.
  */
 export const SETTINGS_SECTIONS: readonly SettingsSection[] = [
   { slug: "account", titleKey: "settings.sections.account.title", icon: User },
@@ -96,11 +104,6 @@ export const SETTINGS_SECTIONS: readonly SettingsSection[] = [
     titleKey: "settings.sections.thresholds.title",
     icon: SlidersHorizontal,
   },
-  {
-    slug: "sources",
-    titleKey: "settings.sections.sources.title",
-    icon: Layers,
-  },
   { slug: "ai", titleKey: "settings.sections.ai.title", icon: Sparkles },
   { slug: "api", titleKey: "settings.sections.api.title", icon: KeyRound },
   {