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.

README.md

@@ -69,10 +69,49 @@ Create `~/.config/terdut-tui/config.yaml`:
 server_url: https://terdut.example.com
 api_key: <your-api-key>
 refresh_interval: 30  # seconds, optional
+theme: gruvbox-dark   # optional, this is the default
 ```
 
 The API key is generated in terdut-server. See the server documentation for how to bootstrap a user and issue an API key.
 
+## Themes
+
+Two themes ship with the client: `gruvbox-dark` (the default) and
+`gruvbox-light`. Both colour foregrounds only — the terminal supplies the
+background — so pick the one that matches the background you already run.
+
+To make your own, drop a file in `~/.config/terdut-tui/themes/` and name it in
+`theme:`. `extends` inherits a built-in, so a file only has to list what it
+changes:
+
+```yaml
+# ~/.config/terdut-tui/themes/mine.yaml
+extends: gruvbox-dark
+primary: "#d3869b"
+accent: "#fabd2f"
+```
+
+A file may shadow a built-in name — `themes/gruvbox-dark.yaml` is how you tweak
+the default without renaming it.
+
+Without `extends`, every token must be set. The twelve are:
+
+| Token | Where it shows |
+|---|---|
+| `primary` | header, active tab, selected row, cursors |
+| `on_primary` | text drawn *on* `primary` — the active tab and selected row |
+| `text` | incident titles and other emphasis |
+| `muted` | secondary text, dividers, footer, table headers |
+| `accent` | status line, acknowledged incidents, the by-day chart |
+| `firing` | firing alerts, triggered incidents, the by-hour chart |
+| `resolved` | resolved alerts and incidents, the top-alerts chart |
+| `error` | error banners |
+| `sev_critical`, `sev_error`, `sev_warning`, `sev_info` | the `severity` label |
+
+Values are hex (`#83a598` or `#abc`) or an ANSI palette index (`0`–`255`) if you
+would rather follow your terminal's own colours. Colours are downsampled
+automatically on 256- and 16-colour terminals, and `NO_COLOR` is honoured.
+
 ## Usage
 
 ```