Collapse the strategy, backtest and sweep panels

Adds a reusable CollapsiblePanel (chevron header, optional summary shown when
collapsed, state persisted in localStorage) and wraps the Strategy, Backtest
and Parameter sweep panels with it, so any single part of the dashboard can be
focused on quickly. Panels stay mounted (v-show) so their form/results state is
preserved across collapse. Includes component tests.

frontend/src/collapsible.test.js

@@ -0,0 +1,64 @@
+import { mount } from '@vue/test-utils'
+import { beforeEach, describe, expect, it } from 'vitest'
+import CollapsiblePanel from './components/CollapsiblePanel.vue'
+
+describe('CollapsiblePanel', () => {
+  beforeEach(() => {
+    localStorage.clear()
+  })
+
+  it('renders the title and slot when open by default', () => {
+    const wrapper = mount(CollapsiblePanel, {
+      props: { title: 'Strategy' },
+      slots: { default: '<p>content</p>' },
+    })
+    expect(wrapper.text()).toContain('Strategy')
+    expect(wrapper.text()).toContain('content')
+    expect(wrapper.find('button').attributes('aria-expanded')).toBe('true')
+  })
+
+  it('hides the slot after clicking the header', async () => {
+    const wrapper = mount(CollapsiblePanel, {
+      props: { title: 'Strategy' },
+      slots: { default: '<p>content</p>' },
+    })
+    await wrapper.find('button').trigger('click')
+    expect(wrapper.find('button').attributes('aria-expanded')).toBe('false')
+    // v-show keeps the node but hides it.
+    expect(wrapper.find('div[style*="display: none"]').exists()).toBe(true)
+  })
+
+  it('shows the summary only when collapsed', async () => {
+    const wrapper = mount(CollapsiblePanel, {
+      props: { title: 'Backtest', summary: 'BTC/USDT 5m' },
+      slots: { default: '<p>content</p>' },
+    })
+    expect(wrapper.text()).not.toContain('BTC/USDT 5m')
+    await wrapper.find('button').trigger('click')
+    expect(wrapper.text()).toContain('BTC/USDT 5m')
+  })
+
+  it('persists the collapsed state in localStorage', async () => {
+    const wrapper = mount(CollapsiblePanel, {
+      props: { title: 'Backtest', storageKey: 'backtest' },
+      slots: { default: '<p>content</p>' },
+    })
+    await wrapper.find('button').trigger('click')
+    expect(localStorage.getItem('panel:backtest')).toBe('0')
+
+    // A fresh mount with the same key starts collapsed.
+    const remounted = mount(CollapsiblePanel, {
+      props: { title: 'Backtest', storageKey: 'backtest' },
+      slots: { default: '<p>content</p>' },
+    })
+    expect(remounted.find('button').attributes('aria-expanded')).toBe('false')
+  })
+
+  it('honours defaultOpen when nothing is stored', () => {
+    const wrapper = mount(CollapsiblePanel, {
+      props: { title: 'Strategy', defaultOpen: false },
+      slots: { default: '<p>content</p>' },
+    })
+    expect(wrapper.find('button').attributes('aria-expanded')).toBe('false')
+  })
+})