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/app/settings/[section]/page.tsx

@@ -11,7 +11,10 @@ import { ExportSection } from "@/components/settings/export-section";
 import { IntegrationsSection } from "@/components/settings/integrations-section";
 import { NotificationsSection } from "@/components/settings/notifications-section";
 import { SectionPlaceholder } from "@/components/settings/section-placeholder";
-import { SourcesSection } from "@/components/settings/sources-section";
+// v1.4.34 IW-D — `sources` slug was retired (merged into `thresholds`).
+// The combined "Targets & Sources" component lives in
+// `thresholds-section.tsx`; `/settings/sources` is now a
+// `permanentRedirect` handled by the dedicated route file.
 import { ThresholdsSection } from "@/components/settings/thresholds-section";
 import {
   SETTINGS_SECTION_SLUGS,
@@ -46,7 +49,6 @@ const SECTION_COMPONENTS: Record<
   notifications: NotificationsSection,
   dashboard: DashboardSection,
   thresholds: ThresholdsSection,
-  sources: SourcesSection,
   api: ApiSection,
   export: ExportSection,
   advanced: AdvancedSection,

src/app/settings/sources/page.tsx

@@ -0,0 +1,23 @@
+import { permanentRedirect } from "next/navigation";
+
+/**
+ * v1.4.34 IW-D — `/settings/sources` permanent redirect.
+ *
+ * The Sources settings section was merged into `/settings/thresholds`
+ * (now titled "Targets & Sources" / "Zielwerte & Quellen"). The two
+ * editors share the same per-metric configuration shelf and reading
+ * them side-by-side on one page matches how a user actually thinks
+ * about metric configuration ("for this metric, which sources do I
+ * trust AND what range counts as healthy?"). Per
+ * `.planning/research/v1434-r-2-carryover-scope.md` §7 and
+ * `.planning/round-v1433-audit-menu.md` §7.1 (item 3).
+ *
+ * 308 (`permanentRedirect`) keeps the request method intact so any
+ * external system that POSTs (none today, but cheap insurance) follows
+ * through with its original method. Existing bookmarks, iOS Settings
+ * deep-links, and any docs that referenced `/settings/sources`
+ * continue to resolve.
+ */
+export default function SettingsSourcesRedirect(): never {
+  permanentRedirect("/settings/thresholds");
+}