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/thresholds-section.tsx

@@ -1,19 +1,47 @@
 "use client";
 
+import { SourcesSection } from "@/components/settings/sources-section";
 import { ThresholdsEditorSection } from "@/components/settings/thresholds-editor-section";
 import { useTranslations } from "@/lib/i18n/context";
 
 /**
  * `<ThresholdsSection>` — route-level wrapper for `/settings/thresholds`.
  *
+ * v1.4.34 IW-D — section was renamed "Zielwerte & Quellen" / "Targets &
+ * Sources" and absorbs the former `/settings/sources` content. The two
+ * editors stay distinct mutation flows (`/api/user/thresholds` for
+ * ranges + `/api/auth/me/source-priority` for source ladders) so a save
+ * on one half never disturbs the other; presenting them on a single
+ * page collapses two adjacent sidebar slots into one and matches the
+ * Settings nav audit (§7.1 item 3 in
+ * `.planning/round-v1433-audit-menu.md` — both screens were per-metric
+ * configuration shelves and reading them together matches how a user
+ * actually thinks about a metric: "for this metric, which sources do
+ * I trust AND what range counts as healthy?").
+ *
+ * `/settings/sources` stays alive as a `permanentRedirect` (see
+ * `src/app/settings/sources/page.tsx`) so external bookmarks (iOS
+ * Settings deep-links, docs) keep resolving.
+ *
+ * Visual structure on this page:
+ *   1. Section header (combined title + description).
+ *   2. `<ThresholdsEditorSection>` — per-metric range inputs (top).
+ *      The editor card kept its own affordance row (icon, reset-all
+ *      button) so the action surface stays compact.
+ *   3. `<SourcesSection>` body — per-metric source priority + the
+ *      two-axis device-type expander (bottom). The inner section's
+ *      header was suppressed in `mode="embedded"` because the page
+ *      header above already names the surface.
+ *
  * v1.4.16 phase B6: file renamed from the historic
- * `thresholds-settings-section.tsx` (`<ThresholdsSettingsSection>`) so the
- * filename + default export match the slug — every other section in
- * `src/components/settings/` follows the `<slug>-section.tsx`
- * `<SlugSection>` convention. The inner editor card was simultaneously
- * moved out of `thresholds-section.tsx` (where it used to live as
- * `<ThresholdsSection>`) into `thresholds-editor-section.tsx`
- * (`<ThresholdsEditorSection>`) so the names stop clashing.
+ * `thresholds-settings-section.tsx` (`<ThresholdsSettingsSection>`)
+ * so the filename + default export match the slug — every other
+ * section in `src/components/settings/` follows the
+ * `<slug>-section.tsx` `<SlugSection>` convention. The inner editor
+ * card was simultaneously moved out of `thresholds-section.tsx`
+ * (where it used to live as `<ThresholdsSection>`) into
+ * `thresholds-editor-section.tsx` (`<ThresholdsEditorSection>`) so
+ * the names stop clashing.
  */
 export function ThresholdsSection() {
   const { t } = useTranslations();
@@ -35,7 +63,15 @@ export function ThresholdsSection() {
         </p>
       </header>
 
+      {/* Per-metric threshold ranges — top card. */}
       <ThresholdsEditorSection id="thresholds" />
+
+      {/* Per-metric source priority + two-axis device-type ladder —
+          bottom card. `mode="embedded"` suppresses the inner h1 +
+          subtitle (the page header above owns the surface name) and
+          drops the inner `<section>` aria-label so the assistive-tech
+          reading order stays one combined surface, not two. */}
+      <SourcesSection mode="embedded" />
     </section>
   );
 }