feat: multi-provider pricing and custom rate overrides

[rajkumarsakthivel]

Summary

• Added static pricing for 15 models across OpenAI, Google, and Anthropic so savings estimates work for non-Claude users
• Added pricing.input and pricing.output config overrides in cce.toml for unlisted or custom-priced models
• Dashboard savings page now shows estimated cost saved, active pricing model, and rates
• Anthropic pricing still auto-fetches from docs (live data wins over static fallbacks)

Example cce.toml configs:

# Use a known model
[pricing]
model = "gpt-4o"

# Override rates for any model
[pricing]
model = "deepseek-v3"
input = 0.27
output = 1.10

Closes #99

[rajkumarsakthivel]

Screenshot

New "Est. Cost Saved" card showing $11.18 saved at opus rates. Pricing note below shows active model and rates with config hint.