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/update_test.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"git.ryuvia.com/niklas/terdut-tui/internal/api"
+	"git.ryuvia.com/niklas/terdut-tui/internal/theme"
 	tea "github.com/charmbracelet/bubbletea"
 )
 
@@ -31,7 +32,7 @@ func press(t *testing.T, m Model, key string) (Model, tea.Cmd) {
 
 // sized returns a connected model with a usable window, which most handlers need.
 func sized() 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
 	return m
@@ -690,7 +691,7 @@ func containsAll(s string, subs ...string) bool {
 // So this test must NOT touch the cursor. It reproduces the real order of
 // events: size first, data second, keys third.
 func TestSchedule_AssignWeekAfterStartupSizingDoesNotPanic(t *testing.T) {
-	m := NewModel(nil, "http://test", time.Minute)
+	m := NewModel(nil, "http://test", time.Minute, theme.GruvboxDark)
 	m.connected = true
 	m.activeSection = sectionSchedule
 	m.scheduleWindow = time.Date(2026, 7, 27, 0, 0, 0, 0, time.UTC)