Themes: text on the accent is its own variable; white on Daylight

Primary buttons hardcoded the dark text that reads on a mint accent. On
Daylight the accent is a deep green, and dark text on it manages 3.6:1 —
squinting territory. The colour that sits on the accent is now
`--accent-text`: dark on the four themes where that reads best (7–10:1),
white on Daylight (4.4:1, the better of the two). Modules can use it too;
the guide lists it with the palette.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/MODULE-GUIDE.md

@@ -377,7 +377,9 @@ own in `stop()` anyway.
 
 2. **Style with the shell's CSS variables — no hardcoded colors.** The
    palette: `--bg` (page), `--panel` (tile), `--border`, `--text`,
-   `--muted`, `--accent`, `--danger`, `--warn`. Derive tints with
+   `--muted`, `--accent`, `--accent-text` (text that sits on the accent —
+   dark on most themes, white where the accent is deep), `--danger`,
+   `--warn`. Derive tints with
    `color-mix(in srgb, var(--accent) 20%, transparent)`. This is what makes
    every module look native — and what makes the dashboard's **themes**
    (Admin → Theme) reach into every tile: a theme swaps those variables and