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.

CLAUDE.md

@@ -28,12 +28,13 @@ non-destructive "not now" alternative.
 main.go                      CLI entry point: flags, config load, health check, start TUI
 internal/api/client.go       REST API client — one method per endpoint
 internal/config/config.go    Config loader (~/.config/terdut-tui/config.yaml)
+internal/theme/              Colour themes: semantic tokens, built-ins, user file loader
 internal/tui/                Bubbletea UI
   model.go                   Model struct, mode/section constants, Init(), tea.Cmd constructors
   update.go                  Update() — dispatch only, no API calls inline
   view.go                    View() — pure rendering
   keys.go                    keyMap (bubbles/key pattern)
-  styles.go                  All lipgloss styles
+  styles.go                  Styles struct — every lipgloss style, built from a theme
 internal/updater/updater.go  Self-update via GitHub Releases
 ```
 
@@ -43,6 +44,9 @@ internal/updater/updater.go  Self-update via GitHub Releases
 2. **`View()` is pure** — no side effects, no state mutations.
 3. **All state in `Model`** — no globals.
 4. **All styles in `styles.go`** — never use lipgloss inline in `view.go`.
+   Styles live on `Model.styles`, built once by `newStyles(theme.Theme)`; the
+   handful of free functions in `view.go` take a `Styles` as their first
+   argument. No colour literal appears outside `internal/theme`.
 
 ## Config
 
@@ -52,8 +56,13 @@ Location: `~/.config/terdut-tui/config.yaml`
 server_url: https://terdut.example.com
 api_key: <64-char hex key>
 refresh_interval: 30  # seconds, optional, default 30
+theme: gruvbox-dark   # optional, default gruvbox-dark
 ```
 
+Built-in themes are `gruvbox-dark` and `gruvbox-light`; user themes are YAML
+files in `~/.config/terdut-tui/themes/`, optionally `extends:`-ing a built-in.
+See the README for the token list.
+
 The API key is a one-time secret generated by terdut-server (`POST /api/users/{id}/api-keys`).
 
 ## Running