fix(server): guard status.html post-build on dflash_server target (unblock CI)

[davide221]

Problem

main fails the CI Build (cmake + uv sync --extra megakernel) → Build dflash (smoke + server) step at configure time:

CMake Warning at CMakeLists.txt:723:  CURL not found — skipping dflash_server (passthrough proxy disabled)
CMake Error   at CMakeLists.txt:748 (add_custom_command):
  No TARGET 'dflash_server' has been created in this directory.

(e.g. run 27104177622, surfaced on #322's CI.)

Root cause — interaction of two already-merged PRs

The CI "Build dflash" job runs without libcurl-dev by design (ci.yml: "Server unit tests require libcurl-dev; skipped when CURL is absent").

• feat(server): passthrough proxy, piecewise keep-ratio curve, query survival check #294 (fix(ci): make CURL optional) made the server target conditional so a no-libcurl box wouldn't break:

  find_package(CURL QUIET)
  if(NOT CURL_FOUND)
      message(WARNING "CURL not found — skipping dflash_server ...")
  else()
      add_executable(dflash_server ...)   # only exists when CURL is found
  endif()

• Add real-time /status dashboard with SSE push #322 (ensure status.html is found from build directory) then added the status.html copy after that endif(), outside the guard:

  add_custom_command(TARGET dflash_server POST_BUILD ...)
  install(FILES ".../status.html" DESTINATION share)

Neither change is wrong alone. Combined on main, when CURL is absent the target is skipped but the post-build copy still references it → hard configure error.

Fix

Wrap the post-build copy + install in if(TARGET dflash_server) so they are only emitted when the target exists. This matches #294's intent (a no-CURL build simply skips the server and its post-build steps).

Verification (clean main checkout, lucebox2, CUDA 12.6)

Reproduced the exact CI error with -DCMAKE_DISABLE_FIND_PACKAGE_CURL=ON on pristine main, then confirmed this patch fixes it:

Scenario	Before	After
CURL disabled (= CI)	No TARGET 'dflash_server' → Configuring incomplete	target skipped → Configuring done
CURL present (= dev)	(unchanged)	dflash_server target created, status.html copied

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