Consolidate TUI debug logging to single dr-tui-debug.log file

[Copilot]

RATIONALE

Multiple TUI components were independently calling tea.LogToFile("tea-debug.log", "debug"), creating fragmented debug logs. Consolidating to a single dr-tui-debug.log file improves debuggability.

CHANGES

• Created tui/logging.go with SetupDebugLogging() helper that returns cleanup function
• Updated cmd/dotenv/cmd.go and cmd/templates/setup/cmd.go to use centralized logging setup
• Changed debug log filename from tea-debug.log to dr-tui-debug.log
• Updated .gitignore accordingly

Before:

f, err := tea.LogToFile("tea-debug.log", "debug")
if err != nil {
    fmt.Println("fatal:", err)
    os.Exit(1)
}
defer f.Close()

After:

cleanup, err := tui.SetupDebugLogging()
if err != nil {
    return err
}
defer cleanup()

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