feat(pricing): remove DeepL from all user-facing surfaces

Pricing: 'deepl' removed from plan provider lists and the comparison
table row. Landing and pricing copy (13 locales) no longer mention
DeepL (65 values cleaned, 39 dead keys deleted: pricing.comparison.deepl,
providerTheme.classic.deepl.*).

Providers: available-provider responses are filtered so a DeepL entry
from the backend can never surface in the engine picker or the services
page; DeepL theme entry and static lib/api.ts list entry removed.

Admin: DeepL config card, type fields, defaults and fallback-chain
mentions removed; stats/chart/status maps and mock data cleaned.

Backend adapter untouched — DeepL is invisible app-wide via the UI
filter; removing the Python adapter itself is a separate step if wanted.

Verified: build exit 0, vitest 9/9, eslint clean on touched files,
zero 'deepl' occurrences outside the two intentional UI filters.

frontend/src/app/admin/useTranslationStats.ts

@@ -78,7 +78,6 @@ function getMockData(period: StatsPeriod): TranslationStatsResponse {
       ],
       provider_breakdown: {
         google: { count: Math.floor(baseCount * 0.476), percentage: 47.6 },
-        deepl: { count: Math.floor(baseCount * 0.357), percentage: 35.7 },
         ollama: { count: Math.floor(baseCount * 0.119), percentage: 11.9 },
         openai: { count: Math.floor(baseCount * 0.048), percentage: 4.8 },
       },