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/batch-import-modal.css

@@ -93,7 +93,7 @@
     content: '\f00c';
     font-family: 'Font Awesome 6 Free', sans-serif;
     font-weight: 900;
-    color: var(--lora-text);
+    color: var(--text-on-accent);
     font-size: 12px;
 }
 
@@ -120,7 +120,7 @@
     white-space: nowrap;
     padding: 8px 16px;
     background: var(--lora-accent);
-    color: var(--lora-text);
+    color: var(--text-on-accent);
     border: none;
     border-radius: var(--border-radius-xs);
     cursor: pointer;
@@ -134,7 +134,7 @@
 /* Dark theme adjustments for input-with-button */
 [data-theme="dark"] .input-with-button button {
     background: var(--lora-accent);
-    color: var(--lora-text);
+    color: var(--text-on-accent);
 }
 
 [data-theme="dark"] .input-with-button button:hover {