feat: add asset inventory page for discovered hosts and services

[aaniya22]

Closes #38

What's changed

• Added AssetInventory.jsx page under frontend/src/pages/ showing all discovered hosts and services
• Added /inventory route in routes.ts and App.tsx
• Added Inventory link to desktop sidebar and mobile nav
• Added GET /api/v1/assets endpoint in backend/secuscan/routes.py
• Wired up getAssets() in api.ts using the existing request() helper

Features

• Table with host/IP, open ports, scanner, finding count, highest severity, first/last seen timestamps, and tags
• Live search filter across host, IP, port, tag, scanner
• Severity and scanner dropdowns
• Sort by host, findings, severity, or last seen
• Click any row to expand a detail panel
• Empty state handled
• Stats strip showing total hosts, unique ports, findings, and last scan time

Testing

• Manually verified with backend running
• Empty state shows correctly when no scans have been run

[utksh1]

Thanks for the asset inventory work. This cannot merge as-is: frontend-checks are failing, the PR removes the Settings route/nav entry, adds an AssetInventory.jsx page with inline/mock-style UI instead of the existing design system, calls /assets without a backend endpoint in this PR, includes unrelated package-lock churn, and bundles the scan-history sidebar changes from another PR. Please split this into focused PRs, restore Settings/Findings behavior, add a real backend/API contract, and make checks green.