feat: add DeepSeek provider with Anthropic-compatible API

[wowitsjack]

Summary

• Adds first-class DeepSeek provider support via their official Anthropic-compatible endpoint (api.deepseek.com/anthropic)
• Maps deepseek-reasoner to Opus (reasoning mode) and deepseek-chat to Sonnet/Haiku (fast mode), both powered by DeepSeek-V3.2
• Includes "DeepSeek Abyss" ocean blue brand theme with custom thinking verbs and phase indicators
• 10-minute API timeout per DeepSeek's recommendation for reasoning tasks

Changes

• src/providers/index.ts — DeepSeek provider definition with model mapping and env config
• src/brands/deepseek.ts — New "DeepSeek Abyss" TweakCC brand theme (ocean blue palette, teal accents)
• src/brands/index.ts — Brand preset registration
• src/tui/content/providers.ts — Education content and comparison list entry
• src/tui/screens/ModelConfigScreen.tsx — Model placeholders for the config screen

Notes

• Uses apiKey auth mode (not authToken), since DeepSeek's Anthropic endpoint expects the key in ANTHROPIC_API_KEY
• DeepSeek-V3.2-Speciale is not available through the Anthropic endpoint, only V3.2

Test plan

• npm run dev -- quick --provider deepseek --api-key <key> creates a working variant
• Variant launches and connects to DeepSeek API successfully
• Both deepseek-reasoner (Opus/thinking mode) and deepseek-chat (Sonnet/Haiku) work
• Brand theme applies correctly (ocean blue colors, thinking verbs)
• npx cc-mirror doctor passes for the deepseek variant
• Existing providers unaffected