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().

templates/layout.php

@@ -39,7 +39,7 @@ $errors = $refresh['errors'] ?? [];
 $total = (int) $page['total'];
 ?>
 <!doctype html>
-<html lang="en" data-bs-theme="light">
+<html lang="en" data-bs-theme="dark">
 <head>
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
@@ -69,7 +69,7 @@ $total = (int) $page['total'];
             <a class="manage-link" href="manage.php">Manage</a>
         </div>
     </div>
-    <div class="spectrum-rail" aria-hidden="true"></div>
+    <div class="wire-rail" aria-hidden="true"></div>
 </header>
 
 <main class="container-xxl py-4">
@@ -151,7 +151,7 @@ $total = (int) $page['total'];
     <div class="text-center mt-5">
         <button id="load-more"
                 type="button"
-                class="btn btn-energy <?= $page['has_more'] ? '' : 'd-none' ?>">
+                class="btn btn-tally <?= $page['has_more'] ? '' : 'd-none' ?>">
             <span class="spinner-border spinner-border-sm me-2 d-none" role="status" aria-hidden="true"></span>
             Load more stories
         </button>