fix(auth): aggregate startup usage notifications

[sunerpy]

Summary

• aggregate per-account startup usage toasts into one summary
• unref the delayed toast timer so short-lived startup processes can exit naturally
• preserve detailed per-account usage logs
• add regression coverage for multi-account startup reporting

Validation

• bun test — 1283 pass, 0 fail
• bun run typecheck
• bun run build
• bunx prettier --check 'src/**/*.ts'
• real default OpenCode route completed with kiro-auth/claude-opus-5-xhigh and finish=stop

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/core/auth/auth-handler.ts	92.19% <100.00%> (+0.60%)	⬆️

... and 1 file with indirect coverage changes

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