Redesign UI color system: neutral ink chrome, sources own the chroma

The light lavender theme carried its color in decoration -- four colored
background blobs, a conic-gradient brand mark, gradient-clipped wordmark,
a four-color animated button, a spinning conic empty mark, and a five-color
spectrum rail. All of that competed with the only color on screen that
carries meaning: which source a story came from.

- One indigo-slate ink ramp (bg / surface / line / ink) carries every neutral
  surface; the only saturated hues are the three sources, used strictly as
  signals -- filter pill, card spine, source chip, timestamp, feed health.
- Rule for every filled state: bright fill, deep ink type. Source tokens kept
  in sync between :root and View::sourceColor().
- Signature "signal bars": a CSS brand mark of three ascending bars in the
  source hues, and the header rail re-cut into three wire segments that double
  as the palette legend. The empty state reuses the mark, unlit and static.
- Bootstrap switched to data-bs-theme="dark" with matching form controls,
  alerts, and focus rings; --ink-faint recalibrated so small text clears
  WCAG 4.5:1 on every surface. Reduced-motion behavior unchanged.
- README documents the two rules next to View::sourceColor().

README.md

@@ -461,6 +461,23 @@ sourceColor('hackernews')  // hex used for the card spine, chip and active pill
 
 The source tab and filter pill are generated automatically from `sources()`.
 
+### Colour system (`assets/style.css`)
+
+The dashboard has **no house colour**. One ink ramp (`--bg` → `--ink`) carries
+every neutral surface, line, and label, and the only saturated colour on screen
+belongs to the sources — used strictly as a signal: filter pill, card spine,
+source chip, timestamp, feed health. Two rules hold it together:
+
+- **Bright fill, deep ink type.** Every filled state (active pill, source chip,
+  primary action) is a bright fill with `--ink-deep` type — no white text on
+  saturated fills, no gradient fills.
+- **One hue per source, mirrored in two places.** `:root` defines the token
+  (`--youtube`, `--reddit`, `--rt`) and `View::sourceColor()` returns the same
+  hex for the inline `--src`/`--pill` values the templates emit. Change both.
+
+The header rail (`.wire-rail`) is the colour legend: one segment per source, in
+order.
+
 ---
 
 ## Deployment notes (Apache + php-fpm on SELinux, e.g. Fedora)