i18n(wallpaper): translate WallpaperPicker via $_() — scope toggle, tabs, sections, upload, overlay

Adds wallpaper.picker namespace covering:
- Scope toggle (Alle Szenen / Nur diese Szene), Reset action
- 3 tabs (Farben/Bilder/Upload) routed via labelKey on the tab data
- Section labels (Empfohlen + Weitere)
- "Hintergrundbilder kommen bald" placeholder for empty images tab
- Upload zone (in-progress, drop, prompt, hint with formats)
- Upload error templates (failed with {status}, generic fallback)
- Loading gallery + "Eigene Bilder" section + delete-image title
- Overlay section + Weichzeichner/Abdunklung labels
- "Bild" alt fallback for media originalName

Baselines: hardcoded 975 → 968 (7 cleared); missing-keys baseline unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/i18n-hardcoded-baseline.json

@@ -34,7 +34,6 @@
 	"apps/mana/apps/web/src/lib/components/shell/ModuleShell.svelte": 3,
 	"apps/mana/apps/web/src/lib/components/SuggestionToast.svelte": 1,
 	"apps/mana/apps/web/src/lib/components/voice/VoiceCaptureBar.svelte": 1,
-	"apps/mana/apps/web/src/lib/components/wallpaper/WallpaperPicker.svelte": 7,
 	"apps/mana/apps/web/src/lib/components/workbench/AppPage.svelte": 6,
 	"apps/mana/apps/web/src/lib/components/workbench/AppPagePicker.svelte": 1,
 	"apps/mana/apps/web/src/lib/components/workbench/SceneAppBar.svelte": 1,