feat(frontend): add wallet session persistence and auto-reconnect

[CMI-James]

Description

Implement wallet session persistence, silent reconnect, extension availability banner, and Soroban invoke guard behavior.

Closes #149

Changes proposed

What were you told to do?

I was tasked with extending wallet session behavior so that:

• Last connected wallet name is persisted and restored
• Silent reconnect is attempted on app load before protected content renders
• Missing wallet extension is surfaced as a persistent banner
• Soroban invoke signing paths are guarded and prompt wallet connect when disconnected
• isConnecting reflects reconnect attempts accurately

What did I do?

Added wallet session persistence and restore

Updated frontend/src/providers/WalletProvider.tsx to:

• Persist wallet provider id in localStorage (payd:last_wallet_name) on connect
• Restore saved wallet id on app startup
• Clear stored wallet id on disconnect

Added silent reconnect on initialization

Provider now:

• Attempts kit.getAddress() when a saved wallet exists
• Sets isConnecting during silent reconnect
• Marks provider as initialized only after reconnect attempt completes
• Delays rendering children until initialization finishes (Restoring wallet session...)

Added extension-availability UX banner

Provider now detects wallet extension presence and shows a persistent warning banner when no supported extension is found.

Added requireWallet guard in wallet context

Extended wallet context API (frontend/src/hooks/useWallet.ts) with:

• requireWallet() guard that prompts connect if disconnected
• isInitialized and walletExtensionAvailable context state

Guarded signing flow against disconnected state

Updated frontend/src/hooks/useWalletSigning.ts to:

• Call requireWallet() before signing
• Prompt user to connect wallet when disconnected
• Surface signing failures via notification
• Expose readiness based on both address and connecting state

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• This PR does not contain plagiarized content.
• The title and description of the PR is clear and explains the approach.
• My commit messages styles matches our requested structure.
• My code additions will fail neither code linting checks nor unit test.
• I am only making changes to files I was requested to.

Screenshots / Testing Evidence

• npm run lint could not be validated in this environment because frontend dependencies are not installed (eslint unavailable)
• npm run build could not be fully validated in this environment due missing node_modules/type packages
• Session restore, reconnect, and guard flow validated through static inspection of provider lifecycle and signing guard paths