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/theme.go

@@ -0,0 +1,32 @@
+// Package theme resolves the named colour scheme the TUI renders with. A theme
+// is a flat set of semantic tokens — roles like "muted" or "firing", never hues
+// — so a new scheme is a table of colours rather than a change to the views.
+package theme
+
+import "github.com/charmbracelet/lipgloss"
+
+// Theme is the palette the UI draws from. Every token is a foreground except
+// OnPrimary, which is the text colour for the two places that invert: the
+// active tab and the selected table row.
+//
+// Colours are truecolor hex; lipgloss downsamples them for 256- and 16-colour
+// terminals and drops them entirely under NO_COLOR, so themes do not carry
+// fallbacks of their own.
+type Theme struct {
+	Name string
+
+	Primary   lipgloss.Color // header, tab highlight, selection
+	OnPrimary lipgloss.Color // text drawn on a Primary background
+	Text      lipgloss.Color // default emphasis foreground
+	Muted     lipgloss.Color // secondary text, dividers, borders
+	Accent    lipgloss.Color // status line, acknowledged, by-day chart
+
+	Firing   lipgloss.Color
+	Resolved lipgloss.Color
+	Error    lipgloss.Color
+
+	SevCritical lipgloss.Color
+	SevError    lipgloss.Color
+	SevWarning  lipgloss.Color
+	SevInfo     lipgloss.Color
+}