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/layout.css

@@ -159,7 +159,7 @@ body.sticky-controls .sticky-topbar {
 
 .control-group button.favorite-filter.active {
     background: var(--lora-accent);
-    color: white;
+    color: var(--text-on-accent);
 }
 
 .control-group button.favorite-filter i {
@@ -174,17 +174,17 @@ body.sticky-controls .sticky-topbar {
 
 .control-group button.update-filter.active {
     background: var(--lora-accent);
-    color: white;
+    color: var(--text-on-accent);
 }
 
 .control-group button.update-filter.active i {
-    color: white;
+    color: var(--text-on-accent);
 }
 
 .update-filter-group .dropdown-main.update-filter.active + .dropdown-toggle {
     background: var(--lora-accent);
     border-color: var(--lora-accent);
-    color: white;
+    color: var(--text-on-accent);
 }
 
 .update-filter-group .dropdown-main.update-filter.active + .dropdown-toggle i {
@@ -194,7 +194,7 @@ body.sticky-controls .sticky-topbar {
 /* Active state for buttons that can be toggled */
 .control-group button.active {
     background: var(--lora-accent);
-    color: white;
+    color: var(--text-on-accent);
     border-color: var(--lora-accent);
 }
 
@@ -540,7 +540,7 @@ body.sticky-controls .sticky-topbar {
 
 .back-to-top:hover {
     background: var(--lora-accent);
-    color: white;
+    color: var(--text-on-accent);
     transform: translateY(-2px);
     box-shadow: var(--shadow-md);
 }