fix(theme): readable on-accent text color and unified accent aliases

Solid-accent buttons used the page text color (--lora-text) or white,
which falls below WCAG AA on the mid-bright accent (L 52-72%) in every
theme/preset — e.g. ~1.7:1 in dark+solarized, making primary buttons
hard to read. Add --color-accent-contrast (dark, low-chroma, same hue,
derived per preset) aliased as --text-on-accent, and switch ~90
accent-background selectors (primary/confirm buttons, nav items, chips,
badges, checkmarks) to it.

Also define the previously dangling --lora-accent-l/c/h, --lora-accent-hover
and --lora-accent-light aliases (fixing ~40 dead declarations), replace the
hardcoded rgba(--lora-accent-rgb, 79 70 229) fallbacks with oklch channel
syntax, and add the missing margin above .template-custom-input in
settings.

static/css/components/sidebar.css

@@ -140,7 +140,7 @@
 }
 
 .sidebar-hidden-indicator:hover i {
-    color: white;
+    color: var(--text-on-accent);
 }
 
 /* Subtle breathing animation for first-time discovery */