Add HAVE_BAUTH conditional compilation for basic auth

[etr]

Summary

• Auto-detect basic auth support in libmicrohttpd via AC_CHECK_LIB for MHD_queue_basic_auth_fail_response
• Guard all basic auth code with #ifdef HAVE_BAUTH / #endif, mirroring the existing HAVE_DAUTH pattern
• Add Basic Auth : yes/no to the ./configure summary output

Addresses item 6 from #61: conditional compilation for libmicrohttpd features (basic auth).

Changes

Build system (configure.ac, src/Makefile.am, examples/Makefile.am):

• Detect MHD_queue_basic_auth_fail_response and set HAVE_BAUTH define + automake conditional
• Conditionally compile basic_auth_fail_response.cpp and install its header
• Conditionally build basic_authentication and centralized_authentication examples

Library sources (18 files, +85/-6 lines):

• basic_auth_fail_response.hpp/.cpp — wrap class/impl in #ifdef HAVE_BAUTH
• http_request.hpp/.cpp — guard get_user(), get_pass(), fetch_user_pass(), cache fields
• create_webserver.hpp — guard basic_auth()/no_basic_auth() builder methods
• webserver.hpp/.cpp — guard basic_auth_enabled member
• httpserver.hpp — conditionally include basic_auth_fail_response.hpp
• create_test_request.hpp/.cpp — guard .user()/.pass() builder and cache access

Tests:

• authentication.cpp, basic.cpp, ws_start_stop.cpp, create_webserver_test.cpp, create_test_request_test.cpp — guard basic auth test cases

Test plan

• All 14 tests pass with HAVE_BAUTH defined (standard libmicrohttpd)
• Source files compile without HAVE_BAUTH (verified via direct compilation)
• Clean build from master with cherry-picked commits passes all tests
• Verify on an embedded system with libmicrohttpd lacking basic auth support

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.97%. Comparing base (1a26dfc) to head (ae59cc7).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #367 +/- ##
==========================================
+ Coverage 67.93% 67.97% +0.03% 
==========================================
Files 28 28 Lines 1634 1636 +2 Branches 672 672 ==========================================
+ Hits 1110 1112 +2 
Misses 64 64 Partials 460 460 

Files with missing lines	Coverage Δ
src/basic_auth_fail_response.cpp	100.00% <ø> (ø)
src/http_request.cpp	60.68% <100.00%> (+0.27%)	⬆️
src/httpserver/basic_auth_fail_response.hpp	75.00% <ø> (ø)
src/httpserver/create_webserver.hpp	96.98% <ø> (ø)
src/httpserver/http_request.hpp	88.37% <ø> (ø)
src/webserver.cpp	56.33% <ø> (ø)

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