refactor(ui): render explicit CaretDown on Select; normalize affordance across platforms

Native <select> chrome varies: macOS renders a dual-arrow indicator,
Windows and Linux render a single caret, and on dark themes the
native caret can drop to near-invisible contrast. The agent Edit
modal (image 5 in the audit) had five side-by-side Selects where the
indicator read as a subtle decoration, making the field look closer
to a filled-in text input than a pick-from-list.

Switched Select to:
- `appearance-none` to strip native chrome
- Phosphor CaretDown rendered absolutely on the right, bold weight,
  muted-foreground so it reads against both card and background
- Padding-right increased to reserve space so long option labels
  do not overlap the caret
- Opacity drop when disabled so the affordance tracks the field state

Scope: only the shared Select primitive; every caller inherits the
fix automatically. No API change.

Covers J10.

src/renderer/components/ui/Select.tsx

@@ -1,3 +1,13 @@
+import { CaretDown } from "./icons";
+
+/**
+ * Native <select> + explicit caret icon. We render our own CaretDown so
+ * the affordance is consistent across platforms (macOS shows dual-arrow,
+ * Windows/Linux a single caret of varying contrast) and visible against
+ * dark themes. `appearance: none` collapses the native chrome so the
+ * caret we render is the only visual indicator; padding-right reserves
+ * space for it so long option labels don't overlap.
+ */
 export function Select({
   value,
   onChange,
@@ -16,22 +26,30 @@ export function Select({
   disabled?: boolean;
 }) {
   const select = (
-    <select
-      className={`focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 w-full max-w-full border bg-card text-foreground ${
-        compact
-          ? "rounded-lg border-border px-2.5 py-1.5 text-[13px] font-medium"
-          : "rounded-lg border-border px-3.5 py-2.5 text-[14px]"
-      } disabled:cursor-not-allowed disabled:opacity-60 truncate`}
-      value={value}
-      disabled={disabled}
-      onChange={(event) => onChange(event.target.value)}
-    >
-      {options.map((option) => (
-        <option key={option.value} value={option.value} className="bg-card text-foreground">
-          {option.label.length > 80 ? option.label.slice(0, 77) + "..." : option.label}
-        </option>
-      ))}
-    </select>
+    <div className="relative">
+      <select
+        className={`focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 w-full max-w-full appearance-none border bg-card text-foreground ${
+          compact
+            ? "rounded-lg border-border pl-2.5 pr-7 py-1.5 text-[13px] font-medium"
+            : "rounded-lg border-border pl-3.5 pr-9 py-2.5 text-[14px]"
+        } disabled:cursor-not-allowed disabled:opacity-60 truncate`}
+        value={value}
+        disabled={disabled}
+        onChange={(event) => onChange(event.target.value)}
+      >
+        {options.map((option) => (
+          <option key={option.value} value={option.value} className="bg-card text-foreground">
+            {option.label.length > 80 ? option.label.slice(0, 77) + "..." : option.label}
+          </option>
+        ))}
+      </select>
+      <CaretDown
+        size={compact ? 12 : 14}
+        weight="bold"
+        className={`pointer-events-none absolute top-1/2 -translate-y-1/2 text-muted-foreground ${compact ? "right-2" : "right-3"} ${disabled ? "opacity-40" : ""}`}
+        aria-hidden="true"
+      />
+    </div>
   );
   if (label) {
     return (