feat: PWA support, collateral endpoints, get_proposed_admin, refinance/extend

[ritik4ever]

closes #865 , closes #854 , closes #853 , closes #856
Changes
#865 — PWA Support (Frontend)

• Added manifest.webmanifest with name, icons (192/512), theme/background colors, display: standalone
• Linked manifest from layout.tsx metadata export
• Added serwist service worker (sw.ts) caching app shell + static assets with stale-while-revalidate; explicitly bypasses /api/ and /sse/ requests
• Added dismissible InstallPrompt component listening for beforeinstallprompt
• Updated next.config.ts with @serwist/next plugin
  [Backend] Expose deposit_collateral and release_collateral via REST endpoints #854 — deposit_collateral / release_collateral REST endpoints (Backend)
• Added buildDepositCollateralTx and buildReleaseCollateralTx to sorobanService.ts
• Added POST /loans/:loanId/build-deposit-collateral and POST /loans/:loanId/build-release-collateral with JWT auth, requireLoanOwner, zod validation, idempotency middleware, and Swagger docs
• Reuses existing POST /loans/submit for the signed XDR submit step
• Unit tests covering success, 403 ownership rejection, and validation
  [Contracts] Add get_proposed_admin view function across LoanManager, LendingPool, RemittanceNFT #853 — get_proposed_admin view function (Contracts)
• Added pub fn get_proposed_admin(env: Env) -> Option to LoanManager, LendingPool, and RemittanceNFT — each bumps instance TTL and reads the ProposedAdmin key
• Unit tests for each contract: no proposal returns None, pending transfer returns Some(address), after accept_admin returns None
  [Backend] Build refinance_loan and extend_loan endpoints #856 — refinance_loan / extend_loan endpoints (Backend)
• Added buildRefinanceLoanTx and buildExtendLoanTx to sorobanService.ts
• Added POST /loans/:loanId/build-refinance and POST /loans/:loanId/build-extend with JWT auth, requireLoanOwner, zod validation, idempotency middleware, and Swagger docs
• Validates newTerm/extraLedgers as positive integers
• Unit tests covering success, 403 ownership rejection, and missing field validation