Colour themes, defaulting to gruvbox dark

Every colour was a 256-colour ANSI index hardcoded in styles.go, so changing
the palette meant editing the styles themselves. This puts a semantic token set
between the two: styles name roles, a theme supplies the colours.

internal/theme holds the twelve tokens, the two built-ins (gruvbox-dark, the
new default, and gruvbox-light) and the loader for user themes in
~/.config/terdut-tui/themes/. A user file may 'extends:' a built-in and
override only what it cares about, and may shadow a built-in name to tweak it
in place. Unknown keys, malformed colours and incomplete themes are refused
with a message naming what went wrong.

Colours are truecolor hex now: lipgloss downsamples for 256- and 16-colour
terminals and honours NO_COLOR, so themes carry no fallbacks of their own.
An ANSI index is still accepted for anyone who would rather follow their
terminal's own palette.

The 21 package-level style vars become a Styles struct on the Model, which is
what rule 3 asked for all along; the four free functions in view.go take one as
their first argument. The embedded bubbles components are restyled from the
same tokens — otherwise a theme would leave a pink selected row and grey help
text behind. Note that the table's Cell style deliberately keeps no foreground:
bubbles renders cells before wrapping the row in Selected, so a colour there
cuts the selection highlight short.

internal/theme/builtin.go

@@ -0,0 +1,88 @@
+package theme
+
+import "sort"
+
+// The gruvbox palettes, in the author's original names. Dark uses the bright
+// variants and light the faded ones, which is what keeps each readable against
+// its own background.
+const (
+	darkBg0  = "#282828"
+	darkFg1  = "#ebdbb2"
+	darkGray = "#928374"
+	darkRed  = "#fb4934"
+	darkGrn  = "#b8bb26"
+	darkYel  = "#fabd2f"
+	darkBlu  = "#83a598"
+	darkAqua = "#8ec07c"
+	darkOrng = "#fe8019"
+
+	lightBg0  = "#fbf1c7"
+	lightFg1  = "#3c3836"
+	lightFg4  = "#7c6f64"
+	lightRed  = "#9d0006"
+	lightGrn  = "#79740e"
+	lightYel  = "#b57614"
+	lightBlu  = "#076678"
+	lightAqua = "#427b58"
+	lightOrng = "#af3a03"
+)
+
+// GruvboxDark is the default scheme. It assumes a dark terminal background:
+// themes colour foregrounds only, so the terminal supplies the canvas.
+var GruvboxDark = Theme{
+	Name: "gruvbox-dark",
+
+	Primary:   darkBlu,
+	OnPrimary: darkBg0,
+	Text:      darkFg1,
+	Muted:     darkGray,
+	Accent:    darkOrng,
+
+	Firing:   darkRed,
+	Resolved: darkGrn,
+	Error:    darkRed,
+
+	SevCritical: darkRed,
+	SevError:    darkOrng,
+	SevWarning:  darkYel,
+	SevInfo:     darkAqua,
+}
+
+// GruvboxLight is the same scheme against a light terminal background.
+var GruvboxLight = Theme{
+	Name: "gruvbox-light",
+
+	Primary:   lightBlu,
+	OnPrimary: lightBg0,
+	Text:      lightFg1,
+	Muted:     lightFg4,
+	Accent:    lightOrng,
+
+	Firing:   lightRed,
+	Resolved: lightGrn,
+	Error:    lightRed,
+
+	SevCritical: lightRed,
+	SevError:    lightOrng,
+	SevWarning:  lightYel,
+	SevInfo:     lightAqua,
+}
+
+// Default is the theme used when the config names none.
+var Default = GruvboxDark
+
+var builtins = map[string]Theme{
+	GruvboxDark.Name:  GruvboxDark,
+	GruvboxLight.Name: GruvboxLight,
+}
+
+// BuiltinNames lists the compiled-in themes, sorted, for error messages and
+// documentation.
+func BuiltinNames() []string {
+	names := make([]string, 0, len(builtins))
+	for name := range builtins {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	return names
+}