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.go

@@ -5,12 +5,12 @@ import (
 	"time"
 
 	"git.ryuvia.com/niklas/terdut-tui/internal/api"
+	"git.ryuvia.com/niklas/terdut-tui/internal/theme"
 	"github.com/charmbracelet/bubbles/help"
 	"github.com/charmbracelet/bubbles/table"
 	"github.com/charmbracelet/bubbles/textinput"
 	"github.com/charmbracelet/bubbles/viewport"
 	tea "github.com/charmbracelet/bubbletea"
-	"github.com/charmbracelet/lipgloss"
 )
 
 // ── Enums ──────────────────────────────────────────────────────────────────
@@ -244,12 +244,14 @@ type Model struct {
 	apiKeyRevokeInput textinput.Model
 	revealedAPIKey    api.APIKey
 
-	help help.Model
-	keys keyMap
+	help   help.Model
+	keys   keyMap
+	styles Styles
 }
 
-func NewModel(client *api.Client, serverURL string, refreshInterval time.Duration) Model {
-	ts := defaultTableStyles()
+func NewModel(client *api.Client, serverURL string, refreshInterval time.Duration, th theme.Theme) Model {
+	st := newStyles(th)
+	ts := st.Table()
 
 	incidentT := table.New(table.WithFocused(true))
 	incidentT.SetStyles(ts)
@@ -301,6 +303,15 @@ func NewModel(client *api.Client, serverURL string, refreshInterval time.Duratio
 	revokeIn.Placeholder = "integer key ID"
 	revokeIn.CharLimit = 20
 
+	for _, in := range []*textinput.Model{
+		&noteIn, &snoozeIn, &usernameIn, &emailIn, &topicIn, &keyNameIn, &revokeIn,
+	} {
+		*in = st.Input(*in)
+	}
+
+	helpModel := help.New()
+	helpModel.Styles = st.Help()
+
 	now := time.Now().UTC()
 	today := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, time.UTC)
 	weekday := int(today.Weekday())
@@ -333,8 +344,9 @@ func NewModel(client *api.Client, serverURL string, refreshInterval time.Duratio
 		ntfyTopicInput:    topicIn,
 		apiKeyNameInput:   keyNameIn,
 		apiKeyRevokeInput: revokeIn,
-		help:              help.New(),
+		help:              helpModel,
 		keys:              keys,
+		styles:            st,
 	}
 }
 
@@ -344,16 +356,6 @@ func (m Model) Init() tea.Cmd {
 
 // ── Table rebuilders ───────────────────────────────────────────────────────
 
-func defaultTableStyles() table.Styles {
-	s := table.DefaultStyles()
-	s.Header = s.Header.Bold(true)
-	s.Selected = s.Selected.
-		Foreground(lipgloss.Color("0")).
-		Background(colorPrimary).
-		Bold(true)
-	return s
-}
-
 // setRows replaces a table's rows and keeps its cursor in a state the rest of
 // this package can rely on: valid whenever the table has any rows at all.
 //
@@ -433,16 +435,16 @@ func (m *Model) refreshDetailContent() {
 		return
 	}
 	if m.mode == modeAlertDetail {
-		m.detailViewport.SetContent(buildAlertDetailContent(m.selectedAlert, m.width))
+		m.detailViewport.SetContent(buildAlertDetailContent(m.styles, m.selectedAlert, m.width))
 		return
 	}
 	m.detailViewport.SetContent(
-		buildIncidentDetailContent(m.selectedIncident, m.timeline, m.noteCursor, m.width))
+		buildIncidentDetailContent(m.styles, m.selectedIncident, m.timeline, m.noteCursor, m.width))
 }
 
 func (m *Model) refreshStatsContent() {
 	m.statsViewport.SetContent(
-		buildStatsContent(m.incidentStats, m.topAlerts, m.hourStats, m.dayStats, m.width))
+		buildStatsContent(m.styles, m.incidentStats, m.topAlerts, m.hourStats, m.dayStats, m.width))
 }
 
 func (m Model) statsViewportHeight() int {