Refactor theme definitions to reduce duplication

[davidnbr]

Description

Both Dashboard and HighContrast themes in getTheme() have nearly identical style definitions (~50 lines duplicated).

Location

main.go:182-238

Suggestion

Extract common styles into a base function:

func baseStyles() Theme {
    return Theme{
        HeaderPlan: lipgloss.NewStyle().Bold(true)...,
        // ... common styles
    }
}

func getTheme(mode RenderingMode) Theme {
    t := baseStyles()
    if mode == RenderingModeHighContrast {
        // Override specific styles only
    }
    t.ErrorReplacer = createGuideReplacer(t.Error)
    t.WarningReplacer = createGuideReplacer(t.Warning)
    return t
}

Impact

• Easier maintenance
• Single source of truth for shared styles
• ~50 lines reduction

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Created from code review of fix_errors_format_default branch