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.

main.go

@@ -7,6 +7,7 @@ import (
 
 	"git.ryuvia.com/niklas/terdut-tui/internal/api"
 	"git.ryuvia.com/niklas/terdut-tui/internal/config"
+	"git.ryuvia.com/niklas/terdut-tui/internal/theme"
 	"git.ryuvia.com/niklas/terdut-tui/internal/tui"
 	"git.ryuvia.com/niklas/terdut-tui/internal/updater"
 	tea "github.com/charmbracelet/bubbletea"
@@ -38,8 +39,14 @@ func main() {
 		os.Exit(1)
 	}
 
+	th, err := theme.Load(cfg.Theme)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
 	client := api.NewClient(cfg.ServerURL, cfg.APIKey)
-	model := tui.NewModel(client, cfg.ServerURL, cfg.RefreshInterval)
+	model := tui.NewModel(client, cfg.ServerURL, cfg.RefreshInterval, th)
 
 	p := tea.NewProgram(model, tea.WithAltScreen())
 	if _, err := p.Run(); err != nil {