feat(ui): add theme toggle

[urzeye]

Summary

• add dark/light theme palette support and persist the selected mode
• add a global T shortcut to toggle themes from the main TUI
• apply theme colors to the main shell, header, list, details, status, hint, and form chrome
• document the new theme toggle in English and Chinese READMEs
• include UI settings and theme mode tests

Tests

• GOCACHE=/private/tmp/lazyssh-gocache go test ./...
• GOCACHE=/private/tmp/lazyssh-gocache GOLANGCI_LINT_CACHE=/private/tmp/lazyssh-golangci-cache ./bin/golangci-lint run
• GOCACHE=/private/tmp/lazyssh-gocache go build -o /private/tmp/lazyssh-theme-check ./cmd

[urzeye]

@codex review

[chatgpt-codex-connector]

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