perf: memoize dashboard streams table and activity list to prevent unnecessary re-renders

[Horlarmmy]

Problem

renderStreams and renderRecentActivity in dashboard-view.tsx were plain functions invoked on every render of DashboardView. Any state change anywhere in this large component (e.g. typing into the template-name input) re-executed the full streams-table and activity-list JSX construction, causing unnecessary re-renders of heavy list components.

Solution

Extracted both render functions into standalone memoized components and stabilized their callback props with useCallback:

1. StreamsTable — Wrapped with React.memo, receives snapshot, onTopUp, onCancel, onShowDetails
2. RecentActivityList — Wrapped with React.memo, receives snapshot, onCreateStream
3. Callback stabilization — Added useCallback hooks for handleTopUp, handleCancel, handleShowDetails, and handleShowWizard to prevent child re-renders from callback identity changes

Changes

• frontend/src/components/dashboard/dashboard-view.tsx
  • Converted renderStreams (lines 369-451) → StreamsTable component with React.memo
  • Converted renderRecentActivity (lines 453-504) → RecentActivityList component with React.memo
  • Added useCallback hooks for 4 callback props in DashboardView
  • Updated 4 call sites from function calls to JSX component usage

Impact

• Streams table and activity list no longer re-render when unrelated local state changes (e.g. templateNameInput, editingTemplateId, selectedTemplateId)
• Reduces wasted render work on the highest-traffic page, especially with larger stream/activity lists
• React DevTools profiler: these components now skip re-renders when snapshot and callbacks remain stable

Testing

All existing tests pass (286/286 across 32 test files). Lint and typecheck clean.

Closes #1243