feat: add TUI theme system with four color palettes

Implement the theming layer for the interactive dashboard:

- tui/theme/theme.go: Theme struct with 14 semantic color roles
  (Background, Surface, Border, BorderHover, TextDim, TextMuted,
  TextPrimary, Accent, Green, Orange, Red, Blue, Purple, Yellow).
  Four built-in themes:

  * Flexoki Dark (default): warm, low-contrast palette designed for
    extended reading. Teal accent (#3AA99F) on near-black background.

  * Catppuccin Mocha: pastel warm tones with blue accent (#89B4FA)
    on dark surface (#1E1E2E).

  * Tokyo Night: cool blue/purple palette with blue accent (#7AA2F7)
    on deep navy (#1A1B26).

  * Terminal: ANSI 16-color only for maximum compatibility — maps
    all roles to standard terminal colors (0-15), works correctly
    in any terminal emulator regardless of true-color support.

  Global Active variable holds the current theme. SetActive/ByName
  enable runtime theme switching from the settings tab and setup
  wizard.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/tui/theme/theme.go

@@ -0,0 +1,119 @@
+package theme
+
+import "github.com/charmbracelet/lipgloss"
+
+// Theme defines the color roles used throughout the TUI.
+type Theme struct {
+	Name        string
+	Background  lipgloss.Color
+	Surface     lipgloss.Color
+	Border      lipgloss.Color
+	BorderHover lipgloss.Color
+	TextDim     lipgloss.Color
+	TextMuted   lipgloss.Color
+	TextPrimary lipgloss.Color
+	Accent      lipgloss.Color
+	Green       lipgloss.Color
+	Orange      lipgloss.Color
+	Red         lipgloss.Color
+	Blue        lipgloss.Color
+	Purple      lipgloss.Color
+	Yellow      lipgloss.Color
+}
+
+// Active is the currently selected theme.
+var Active = FlexokiDark
+
+// FlexokiDark is the default theme.
+var FlexokiDark = Theme{
+	Name:        "flexoki-dark",
+	Background:  lipgloss.Color("#100F0F"),
+	Surface:     lipgloss.Color("#1C1B1A"),
+	Border:      lipgloss.Color("#282726"),
+	BorderHover: lipgloss.Color("#343331"),
+	TextDim:     lipgloss.Color("#575653"),
+	TextMuted:   lipgloss.Color("#6F6E69"),
+	TextPrimary: lipgloss.Color("#FFFCF0"),
+	Accent:      lipgloss.Color("#3AA99F"),
+	Green:       lipgloss.Color("#879A39"),
+	Orange:      lipgloss.Color("#DA702C"),
+	Red:         lipgloss.Color("#D14D41"),
+	Blue:        lipgloss.Color("#4385BE"),
+	Purple:      lipgloss.Color("#8B7EC8"),
+	Yellow:      lipgloss.Color("#D0A215"),
+}
+
+// CatppuccinMocha is a warm pastel theme.
+var CatppuccinMocha = Theme{
+	Name:        "catppuccin-mocha",
+	Background:  lipgloss.Color("#1E1E2E"),
+	Surface:     lipgloss.Color("#313244"),
+	Border:      lipgloss.Color("#45475A"),
+	BorderHover: lipgloss.Color("#585B70"),
+	TextDim:     lipgloss.Color("#6C7086"),
+	TextMuted:   lipgloss.Color("#A6ADC8"),
+	TextPrimary: lipgloss.Color("#CDD6F4"),
+	Accent:      lipgloss.Color("#89B4FA"),
+	Green:       lipgloss.Color("#A6E3A1"),
+	Orange:      lipgloss.Color("#FAB387"),
+	Red:         lipgloss.Color("#F38BA8"),
+	Blue:        lipgloss.Color("#89B4FA"),
+	Purple:      lipgloss.Color("#CBA6F7"),
+	Yellow:      lipgloss.Color("#F9E2AF"),
+}
+
+// TokyoNight is a cool blue/purple theme.
+var TokyoNight = Theme{
+	Name:        "tokyo-night",
+	Background:  lipgloss.Color("#1A1B26"),
+	Surface:     lipgloss.Color("#24283B"),
+	Border:      lipgloss.Color("#414868"),
+	BorderHover: lipgloss.Color("#565F89"),
+	TextDim:     lipgloss.Color("#565F89"),
+	TextMuted:   lipgloss.Color("#A9B1D6"),
+	TextPrimary: lipgloss.Color("#C0CAF5"),
+	Accent:      lipgloss.Color("#7AA2F7"),
+	Green:       lipgloss.Color("#9ECE6A"),
+	Orange:      lipgloss.Color("#FF9E64"),
+	Red:         lipgloss.Color("#F7768E"),
+	Blue:        lipgloss.Color("#7AA2F7"),
+	Purple:      lipgloss.Color("#BB9AF7"),
+	Yellow:      lipgloss.Color("#E0AF68"),
+}
+
+// Terminal uses ANSI 16 colors only.
+var Terminal = Theme{
+	Name:        "terminal",
+	Background:  lipgloss.Color("0"),
+	Surface:     lipgloss.Color("0"),
+	Border:      lipgloss.Color("8"),
+	BorderHover: lipgloss.Color("7"),
+	TextDim:     lipgloss.Color("8"),
+	TextMuted:   lipgloss.Color("7"),
+	TextPrimary: lipgloss.Color("15"),
+	Accent:      lipgloss.Color("6"),
+	Green:       lipgloss.Color("2"),
+	Orange:      lipgloss.Color("3"),
+	Red:         lipgloss.Color("1"),
+	Blue:        lipgloss.Color("4"),
+	Purple:      lipgloss.Color("5"),
+	Yellow:      lipgloss.Color("3"),
+}
+
+// All available themes.
+var All = []Theme{FlexokiDark, CatppuccinMocha, TokyoNight, Terminal}
+
+// ByName returns a theme by its name, defaulting to FlexokiDark.
+func ByName(name string) Theme {
+	for _, t := range All {
+		if t.Name == name {
+			return t
+		}
+	}
+	return FlexokiDark
+}
+
+// SetActive sets the active theme by name.
+func SetActive(name string) {
+	Active = ByName(name)
+}