refactor(theme): the accent was hand-written 16 times — now derived

`rgba(91, 74, 138, …)` is Scribe's accent in decimal. It appears sixteen times
across five files at ten different opacities, plus once as #5B4A8A. Change the
accent in the design system and none of them would have moved — which is the
precise failure the token system exists to prevent, hiding in a notation that
does not look like a colour constant.

Now `color-mix(in srgb, var(--color-primary) N%, transparent)`, so every one
follows the accent. The design system already uses this form for its own tints
(--fs-accent-soft, -faint, -wash), so this is the established idiom rather than
a new one.

The CI literal count barely moves (45 -> 44) because its regex matches #hex and
fifteen of these were rgba(). Worth stating plainly: **the count was never the
goal, and the check is blind to this whole class.** An rgba triple is a colour
literal in every sense that matters and the report does not see it.

Not touched: the badge palette in KnowledgeView (#7A6DA8, #fbbf24, #818cf8 for
note/task/plan) and the remaining greys. Those are genuine unmade decisions —
what colour IS a plan badge — not drift, and inventing tokens for them would be
deciding by implementation.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/views/SettingsView.vue

@@ -2404,7 +2404,7 @@ function formatUserDate(iso: string): string {
 }
 .sidebar-item.active {
   color: var(--color-primary);
-  background: rgba(91, 74, 138, 0.08);
+  background: color-mix(in srgb, var(--color-primary) 8%, transparent);
   border-left-color: var(--color-primary);
   font-weight: 500;
 }