feat(ui): add resizable sidebar panels with persistence

- use-panel-resize hook: Drag-to-resize with localStorage persistence
- ResizeHandle component: Visual drag handle with hover effects
- UnifiedShell: Flexbox layout with resizable left/right panels
- RightPanel: Removed internal width handling (now controlled by shell)

src/hooks/use-panel-resize.ts

@@ -0,0 +1,61 @@
+import { useState, useEffect, useCallback } from 'react';
+
+const LEFT_PANEL_KEY = 'bb.ui.leftPanelWidth';
+const RIGHT_PANEL_KEY = 'bb.ui.rightPanelWidth';
+
+const DEFAULT_LEFT_WIDTH = 320;
+const DEFAULT_RIGHT_WIDTH = 332;
+
+export const MIN_LEFT_WIDTH = 192;
+export const MIN_RIGHT_WIDTH = 256;
+
+export function usePanelResize() {
+    const [leftWidth, setLeftWidth] = useState(() => {
+        if (typeof window === 'undefined') return DEFAULT_LEFT_WIDTH;
+        const saved = localStorage.getItem(LEFT_PANEL_KEY);
+        return saved ? parseInt(saved, 10) : DEFAULT_LEFT_WIDTH;
+    });
+
+    const [rightWidth, setRightWidth] = useState(() => {
+        if (typeof window === 'undefined') return DEFAULT_RIGHT_WIDTH;
+        const saved = localStorage.getItem(RIGHT_PANEL_KEY);
+        return saved ? parseInt(saved, 10) : DEFAULT_RIGHT_WIDTH;
+    });
+
+    useEffect(() => {
+        localStorage.setItem(LEFT_PANEL_KEY, String(leftWidth));
+    }, [leftWidth]);
+
+    useEffect(() => {
+        localStorage.setItem(RIGHT_PANEL_KEY, String(rightWidth));
+    }, [rightWidth]);
+
+    const clampLeftWidth = useCallback((width: number) => {
+        const maxWidth = Math.floor(window.innerWidth * 0.30);
+        return Math.max(MIN_LEFT_WIDTH, Math.min(width, maxWidth));
+    }, []);
+
+    const clampRightWidth = useCallback((width: number) => {
+        const maxWidth = Math.floor(window.innerWidth * 0.35);
+        return Math.max(MIN_RIGHT_WIDTH, Math.min(width, maxWidth));
+    }, []);
+
+    const handleLeftResize = useCallback((delta: number) => {
+        setLeftWidth(prev => clampLeftWidth(prev + delta));
+    }, [clampLeftWidth]);
+
+    const handleRightResize = useCallback((delta: number) => {
+        setRightWidth(prev => clampRightWidth(prev + delta));
+    }, [clampRightWidth]);
+
+    return {
+        leftWidth,
+        rightWidth,
+        handleLeftResize,
+        handleRightResize,
+        clampLeftWidth,
+        clampRightWidth,
+        MIN_LEFT_WIDTH,
+        MIN_RIGHT_WIDTH
+    };
+}