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/config/config.go

@@ -15,12 +15,14 @@ type Config struct {
 	ServerURL       string
 	APIKey          string
 	RefreshInterval time.Duration
+	Theme           string
 }
 
 type rawConfig struct {
 	ServerURL       string `yaml:"server_url"`
 	APIKey          string `yaml:"api_key"`
 	RefreshInterval int    `yaml:"refresh_interval,omitempty"` // seconds
+	Theme           string `yaml:"theme,omitempty"`
 }
 
 func Load() (*Config, error) {
@@ -33,7 +35,7 @@ func Load() (*Config, error) {
 	data, err := os.ReadFile(path)
 	if err != nil {
 		if os.IsNotExist(err) {
-			return nil, fmt.Errorf("config file not found at %s\n\nCreate it with:\n  server_url: https://terdut.example.com\n  api_key: <your-api-key>", path)
+			return nil, fmt.Errorf("config file not found at %s\n\nCreate it with:\n  server_url: https://terdut.example.com\n  api_key: <your-api-key>\n  theme: gruvbox-dark  # optional", path)
 		}
 		return nil, fmt.Errorf("cannot read config file: %w", err)
 	}
@@ -59,5 +61,6 @@ func Load() (*Config, error) {
 		ServerURL:       raw.ServerURL,
 		APIKey:          raw.APIKey,
 		RefreshInterval: interval,
+		Theme:           raw.Theme,
 	}, nil
 }