feat: custom-styled checkboxes + select dropdowns

Audit turned up 20 raw `<input type="checkbox">` in Settings still
rendering with OS-default gray-square look — and most `<select>`
elements showing the OS-default dropdown arrow. With the rest of the
UI now Twitch-themed (purple inputs, modal pops, animated everything),
those felt jarringly out of place.

Checkbox: 16px rounded square, dark base with 1.5px border, hovers to
a purple-tinted border, fills purple + draws a CSS-only white check
on the diagonal when checked, scales down briefly on click, focus
shows the same 3px purple ring as the text inputs. No JS, just
:checked + ::after.

Select: appearance:none everywhere to kill the OS chevron, then an
inline-SVG chevron in background-image at right:8px (gray default,
purple on hover). padding-right boosted to 28px so option text never
overlaps the arrow. The dropdown menu itself still uses the OS list,
but the closed control matches the rest of the input family now.

Cascades globally via input[type="checkbox"] / select selectors — no
markup edits needed. The few selects/checkboxes that previously had
inline accent-color overrides keep working because accent-color
applies to native widgets, which we have now replaced.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/styles.css

@@ -314,6 +314,86 @@ select:focus {
     box-shadow: 0 0 0 3px rgba(145, 70, 255, 0.18);
 }
 
+/* ============================================
+   CUSTOM CHECKBOX — modern Twitch-purple
+   ============================================
+   Replaces the OS gray box with a 16px rounded square that fills
+   purple + draws a white check when checked. Smooth 0.18s transition
+   on every state change. Focus ring matches inputs. */
+input[type="checkbox"] {
+    appearance: none;
+    -webkit-appearance: none;
+    width: 16px;
+    height: 16px;
+    flex-shrink: 0;
+    border: 1.5px solid var(--border-soft);
+    border-radius: 4px;
+    background: var(--bg-card);
+    cursor: pointer;
+    position: relative;
+    transition: background 0.18s, border-color 0.18s, box-shadow 0.18s, transform 0.12s;
+    vertical-align: middle;
+}
+
+input[type="checkbox"]:hover:not(:disabled) {
+    border-color: rgba(145, 70, 255, 0.6);
+}
+
+input[type="checkbox"]:checked {
+    background: var(--accent);
+    border-color: var(--accent);
+}
+
+input[type="checkbox"]:checked::after {
+    content: '';
+    position: absolute;
+    left: 4px;
+    top: 0.5px;
+    width: 5px;
+    height: 9px;
+    border: solid #fff;
+    border-width: 0 2px 2px 0;
+    transform: rotate(45deg);
+}
+
+input[type="checkbox"]:focus-visible {
+    outline: none;
+    box-shadow: 0 0 0 3px rgba(145, 70, 255, 0.25);
+}
+
+input[type="checkbox"]:active:not(:disabled) {
+    transform: scale(0.92);
+}
+
+input[type="checkbox"]:disabled {
+    opacity: 0.45;
+    cursor: not-allowed;
+}
+
+/* ============================================
+   CUSTOM SELECT — chevron via inline SVG background
+   ============================================ */
+select {
+    appearance: none;
+    -webkit-appearance: none;
+    -moz-appearance: none;
+    background-image: url("data:image/svg+xml;charset=utf-8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' fill='%23999'%3E%3Cpath d='M4 6l4 4 4-4z'/%3E%3C/svg%3E");
+    background-repeat: no-repeat;
+    background-position: right 8px center;
+    background-size: 14px;
+    padding-right: 28px !important;
+    cursor: pointer;
+}
+
+select:hover:not(:disabled) {
+    background-image: url("data:image/svg+xml;charset=utf-8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' fill='%239146FF'%3E%3Cpath d='M4 6l4 4 4-4z'/%3E%3C/svg%3E");
+}
+
+select option {
+    background: var(--bg-card);
+    color: var(--text);
+}
+
 /* Queue Section */
 .queue-section {
     border-top: 1px solid rgba(255,255,255,0.1);