fix(chat): update context usage while streaming

[musnows]

Summary

• Update the context usage ring from live conversation blocks while a turn is streaming.
• Keep provider prompt-token usage as calibration, but let the local live estimate raise stale measured totals during streaming.
• Add regression coverage for live estimates exceeding an older measured total.

Tests

• npm run test -- src/renderer/src/lib/context-capacity.test.ts
• npm run typecheck
• git diff --check