refactor(cli): fix timer leak, remove dead code, and consolidate error parsing

[appleboy]

Summary

• Fix time.After timer leak in callback server — replaced with time.NewTimer + defer timer.Stop() to prevent 2-minute goroutine leaks on every successful OAuth callback
• Remove unnecessary time.Sleep(100ms) in browser flow and device flow error paths — the buffered channel and consumer drain loops already ensure updates are processed
• Remove 4 unused functions (GetFloat64, FlowUpdateType.String(), FormatDurationCompact, FormatInterval) and their associated test code
• Extract parseOAuthError helper to consolidate triplicated OAuth error JSON unmarshaling across formatHTTPError, doTokenExchange, and handleDevicePollError
• Delegate error formatting in doTokenExchange to formatHTTPError to eliminate duplicate formatting logic

Test plan

• All existing tests pass (make test)
• Linter passes with 0 issues (make lint)
• Code formatted (make fmt)
• Verify no regressions in OAuth browser flow (PKCE) end-to-end
• Verify no regressions in device code flow end-to-end

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 37.50000% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tokens.go	27.27%	7 Missing and 1 partial ⚠️
callback.go	66.66%	1 Missing ⚠️
device_flow.go	50.00%	0 Missing and 1 partial ⚠️

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