feat: refactor owner-portal into modular component architecture

[PranavGadge01]

Summary

Refactors the owner-portal from a single monolithic component into a
properly structured React application with separated concerns.

Motivation

The entire portal was contained in two files (OwnerDashboard.jsx + CSS),
making it hard to maintain, extend, and test. This refactor applies
standard React architecture patterns.

New Structure

src/
├── components/
│   ├── layout/
│   │   ├── Sidebar.jsx        # Navigation sidebar
│   │   └── Sidebar.css
│   ├── tabs/
│   │   ├── Overview.jsx       # Wallet stats and account info
│   │   ├── MintNFT.jsx        # NFT minting form
│   │   ├── History.jsx        # Transaction history
│   │   └── Settings.jsx       # App settings
│   ├── ui/
│   │   ├── StatCard.jsx       # Reusable stat display card
│   │   ├── Alert.jsx          # Inline alert component
│   │   └── Toast.jsx          # Auto-dismissing notifications
├── constants/
│   └── tabs.js                # Tab definitions
├── hooks/
│   └── useSessionSigner.js    # Session signer logic extracted
└── utils/
    └── validators.js          # IPFS/hash/signature validators

Changes

• Extracted session signer logic into useSessionSigner custom hook
• Split UI into 4 tabs: Overview, Mint NFT, History, Settings
• Created reusable StatCard, Alert, and Toast UI components
• Moved validation functions to utils/validators.js
• Tab constants extracted to constants/tabs.js
• OwnerDashboard.jsx now only handles layout and tab routing

Testing

• App loads and authenticates successfully
• All 4 tabs navigate correctly
• Wallet connection displays in sidebar
• Mint form validates inputs before submission
• Session signer error handled gracefully when backend is offline

Screenshots

Dashboard Overview

Mint NFT Tab