Move color mode selector to elevation chart, add grade profile

- Moved color mode dropdown from header bar to inline with the
  elevation chart title — closer to the visualization it controls
- Removed ColorModeToggle component from SessionView header
- Added "Grade" color mode: colors route and elevation chart by
  steepness (green <3%, yellow <6%, orange <10%, red <15%, dark red 15%+)
- Grade hover label shows percentage (e.g. "+4.2%")
- Fixed useTranslation namespace in ElevationChart ("planner")
- i18n: Added grade keys for en + de

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/i18n/src/locales/de.ts

@@ -101,6 +101,7 @@ export default {
       plain: "Standard",
       elevation: "Höhe",
       surface: "Untergrund",
+      grade: "Steigung",
       surfaceUnavailable: "Untergrunddaten für dieses Profil nicht verfügbar",
     },
     rateLimitExceeded: "Zu viele Anfragen. Bitte versuche es später erneut.",
@@ -108,6 +109,7 @@ export default {
       gain: "Höhenmeter aufwärts",
       loss: "Höhenmeter abwärts",
       profile: "Höhenprofil",
+      grade: "Steigungsprofil",
     },
     landing: {
       startPlanning: "Planung starten",

packages/i18n/src/locales/en.ts

@@ -101,6 +101,7 @@ export default {
       plain: "Plain",
       elevation: "Elevation",
       surface: "Surface",
+      grade: "Grade",
       surfaceUnavailable: "Surface data not available for this profile",
     },
     rateLimitExceeded: "Too many requests. Please try again later.",
@@ -108,6 +109,7 @@ export default {
       gain: "Elevation Gain",
       loss: "Elevation Loss",
       profile: "Elevation Profile",
+      grade: "Grade Profile",
     },
     landing: {
       startPlanning: "Start Planning",