feat(ui): implement pills-style AppSwitcher with consistent button widths

Replace segmented control with pills-style switcher for better visual consistency.

src/components/AppSwitcher.css

@@ -0,0 +1,66 @@
+/* 药丸式切换按钮 */
+.switcher-pills {
+  display: inline-flex;
+  align-items: center;
+  gap: 12px;
+  background: rgba(255, 255, 255, 0.08);
+  padding: 6px 8px;
+  border-radius: 50px;
+  backdrop-filter: blur(10px);
+}
+
+.switcher-pill {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: 8px;
+  padding: 8px 16px;
+  background: transparent;
+  border: none;
+  border-radius: 50px;
+  color: rgba(255, 255, 255, 0.6);
+  font-size: 14px;
+  font-weight: 600;
+  cursor: pointer;
+  transition: all 200ms ease;
+  min-width: 120px;
+}
+
+.switcher-pill:hover:not(.active) {
+  color: rgba(255, 255, 255, 0.8);
+  background: rgba(255, 255, 255, 0.05);
+}
+
+.switcher-pill.active {
+  background: rgba(255, 255, 255, 0.15);
+  color: white;
+  box-shadow: 
+    inset 0 1px 3px rgba(0, 0, 0, 0.1),
+    0 1px 0 rgba(255, 255, 255, 0.1);
+}
+
+.pill-dot {
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  background: currentColor;
+  opacity: 0.4;
+  transition: all 200ms ease;
+}
+
+.switcher-pill.active .pill-dot {
+  opacity: 1;
+  box-shadow: 0 0 8px currentColor;
+  animation: pulse 2s infinite;
+}
+
+.pills-divider {
+  width: 1px;
+  height: 20px;
+  background: rgba(255, 255, 255, 0.2);
+}
+
+@keyframes pulse {
+  0%, 100% { transform: scale(1); opacity: 1; }
+  50% { transform: scale(1.2); opacity: 0.8; }
+}

src/components/AppSwitcher.tsx

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+import { AppType } from "../lib/tauri-api";
+import "./AppSwitcher.css";
+
+interface AppSwitcherProps {
+  activeApp: AppType;
+  onSwitch: (app: AppType) => void;
+}
+
+export function AppSwitcher({ activeApp, onSwitch }: AppSwitcherProps) {
+  const handleSwitch = (app: AppType) => {
+    if (app === activeApp) return;
+    onSwitch(app);
+  };
+
+  return (
+    <div className="switcher-pills">
+      <button
+        type="button"
+        className={`switcher-pill ${activeApp === "claude" ? "active" : ""}`}
+        onClick={() => handleSwitch("claude")}
+      >
+        <span className="pill-dot" />
+        <span>Claude Code</span>
+      </button>
+      <div className="pills-divider" />
+      <button
+        type="button"
+        className={`switcher-pill ${activeApp === "codex" ? "active" : ""}`}
+        onClick={() => handleSwitch("codex")}
+      >
+        <span className="pill-dot" />
+        <span>Codex</span>
+      </button>
+    </div>
+  );
+}