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/tui/model_test.go

@@ -5,6 +5,7 @@ import (
 	"time"
 
 	"git.ryuvia.com/niklas/terdut-tui/internal/api"
+	"git.ryuvia.com/niklas/terdut-tui/internal/theme"
 	"github.com/charmbracelet/bubbles/table"
 )
 
@@ -177,7 +178,7 @@ func TestAlertRows_ShowIncidentLink(t *testing.T) {
 func TestUserManageRows_ShowMissingTopic(t *testing.T) {
 	topic := "terdut-niklas"
 	empty := ""
-	m := NewModel(nil, "http://test", time.Minute)
+	m := NewModel(nil, "http://test", time.Minute, theme.GruvboxDark)
 	m.width, m.height = 120, 40
 	m.users = []api.User{
 		{ID: 1, Username: "niklas", NtfyTopic: &topic},
@@ -283,7 +284,7 @@ func TestBuildScheduleDays(t *testing.T) {
 // scheduledWeek builds a model showing the week of 2026-07-27 with the given
 // entries already on the rota.
 func scheduledWeek(entries []api.ScheduleEntry) Model {
-	m := NewModel(nil, "http://test", time.Minute)
+	m := NewModel(nil, "http://test", time.Minute, theme.GruvboxDark)
 	m.width, m.height = 120, 40
 	m.connected = true
 	m.activeSection = sectionSchedule