fix: complete thinking streams and canonicalize model IDs

[goodnightzsj]

Summary

• emit a non-empty signature_delta before every thinking block stop and preserve real signatures in reconstructed message responses
• merge OpenAI-compatible finish reasons and trailing usage into one terminal Anthropic message_delta
• canonicalize uniquely matched built-in model IDs case-insensitively while preserving exact custom IDs and ambiguity errors
• keep Kimi request thinking parameters capability-gated, with a regression covering thinking history

Validation

• go test ./... -count=1
• go vet ./...
• test -z "$(gofmt -l cmd internal pkg)"
• node --check internal/gui/assets/app.js

Fork scope

The source fork also has SQLite request history, provider usage reconciliation, expanded usage dashboards, themed controls, and public deployment work. Those fork-specific changes are deliberately excluded; this PR contains one upstream-based commit with only the generally applicable protocol and model-resolution fixes.

Fixes #51
Fixes #131