feat(accent): custom accent themes links, text selection, and focus rings

The accent previously only overrode the folds Primary.* family; links kept the
hardcoded --tc-link blue, ::selection was browser-default, and focus rings were
neutral grey (Other.FocusRing). Now all three derive from the chosen base color:
- --tc-link → accent hex (messages, topics, URL previews)
- ::selection via an injected <style id="lotus-accent-style"> (accent bg +
  WCAG-contrasting text)
- Other.FocusRing → rgba(accent, 0.5)

Deliberately NOT recolored: Secondary.* (doubles as the neutral text/button/
badge palette), Success.* + mention pills (semantic mention/notification green),
scrollbar thumbs (folds styles them per-component; a global rule would only
half-apply). removeCustomAccent() clears everything — no residue when switching
off or to the TDS theme. +2 unit tests (561 total).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/utils/accentColor.test.ts

@@ -9,6 +9,8 @@ import {
   contrastingText,
   varNameFromToken,
   derivePrimaryPalette,
+  deriveAccentExtras,
+  buildAccentCss,
 } from './accentColor';
 
 test('hexToRgb parses 6-digit hex (with/without #, trimmed)', () => {
@@ -66,3 +68,22 @@ test('derivePrimaryPalette produces the full Primary token set', () => {
   assert.match(palette.MainHover, /^#[0-9a-f]{6}$/);
   assert.match(palette.MainActive, /^#[0-9a-f]{6}$/);
 });
+
+test('deriveAccentExtras derives focus ring, link and selection from one base', () => {
+  const base = { r: 255, g: 136, b: 0 };
+  const extras = deriveAccentExtras(base);
+  // focus ring keeps the translucent character in the accent hue
+  assert.equal(extras.focusRing, 'rgba(255, 136, 0, 0.5)');
+  // link + selection background are the solid base hex
+  assert.equal(extras.link, '#ff8800');
+  assert.equal(extras.selectionBg, '#ff8800');
+  // selection text is WCAG-aware contrasting text over the base
+  assert.equal(extras.selectionText, contrastingText(base));
+});
+
+test('buildAccentCss emits selection rules using the derived palette', () => {
+  const base = { r: 0, g: 0, b: 0 };
+  const css = buildAccentCss(base);
+  assert.match(css, /::selection\{background:#000000;color:#fff;\}/);
+  assert.match(css, /::-moz-selection\{background:#000000;color:#fff;\}/);
+});