feat(backend): add centralized error handling

[shoaib050326]

## PR Title
feat(backend): add centralized error handling middleware

## PR Description

### Summary
Adds centralized error handling to the backend so unhandled route errors are caught consistently, returned in a standard response format, and masked appropriately in production.

### Changes
- add a new `backend/src/middleware/errorHandler.ts` with:
  - `AppError` for typed application errors
  - `asyncHandler` for async route wrapping
  - `notFoundHandler` for unknown routes
  - `errorHandler` for standardized error responses
- wire the global error middleware into `backend/src/index.ts`
- refactor backend routes to throw `AppError` instead of returning scattered ad hoc 500 responses
- keep route-level validation failures as `400` and missing resources as `404`
- log errors with severity based on status code
- hide internal error details in production while keeping useful debug context in development
- fix route param typing issues uncovered during verification
- update the Stellar validation test with a valid sample public key
- clear a lint error in `backend/src/services/catalog.ts`

### Validation
- `cd backend && npm ci`
- `cd backend && npm run build`
- `cd backend && npm run lint`
- `cd backend && npm test`

### Related Issue
Closes #13

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[shoaib050326]

## Update

I investigated the repeated frontend CI failures on this PR and found that they were coming from a dependency mismatch rather than a single isolated code bug.

### Root Cause
The frontend had incompatible Web3Auth packages installed together:
- `@web3auth/modal` was on `^10.15.0`
- `@web3auth/base` and `@web3auth/ethereum-provider` were still on `^8.12.4`

Because of that mismatch, CI kept surfacing different TypeScript errors in `frontend/components/auth/SocialLogin.tsx` one after another, depending on which package surface TypeScript resolved first.

### What I Changed
- aligned the Web3Auth packages to a single compatible version line:
  - `@web3auth/base@^9.7.0`
  - `@web3auth/ethereum-provider@^9.7.0`
  - `@web3auth/modal@^9.7.0`
- updated `frontend/package-lock.json` to match the corrected dependency set
- updated `frontend/components/auth/SocialLogin.tsx` to use the matching v9 API:
  - `web3auth.initModal()`
  - `WALLET_ADAPTERS.AUTH`

### Other CI Fixes Already Included In This Branch
This branch also includes the earlier fixes for:
- backend centralized error handling
- contract build failure from missing `deadline` in batch project creation
- frontend invoice/verification typing issue
- frontend wagmi import issue that was pulling optional connector packages into the build
- frontend font/build stability adjustments

### Validation
Confirmed locally after the dependency alignment:
- `cargo build --release` ✅
- `npm run build` / frontend code path verified
- `npx next build --webpack` ✅

[Smartdevs17]

resolve conflicts