Add a 'Rebuild llama.cpp' Cookbook action to force a fresh GPU build (#1787)

The serve bootstrap builds llama-server from source only when it is missing
from PATH, so a host that first compiled CPU-only (no nvcc present at build
time) reuses that CPU-only binary on every later serve and never gets a GPU
build, even after a CUDA/ROCm toolkit is installed. There was no UI lever to
force a rebuild.

Adds a 'Rebuild llama.cpp' button to the Cookbook Dependencies tab. It clears
the cached ~/bin/llama-server symlink and ~/llama.cpp/build directory (locally
or on the selected remote server) so the next serve recompiles and picks up
CUDA/HIP if a toolchain is now present. It installs and downloads nothing.

- routes/cookbook_helpers.py: _llama_cpp_rebuild_cmd() (single source of truth)
- routes/shell_routes.py: POST /api/cookbook/rebuild-engine (admin-only, reuses
  the existing SSH plumbing for remote hosts)
- static/js/cookbook.js: header button + handler honoring the deps server selector
- tests: cover the command shape and a clean run on a fresh HOME

Motivated by #831 (RTX 4070 user stuck on a CPU-only build with no way to
re-trigger the build).

Co-authored-by: ghreprimand <203024559+ghreprimand@users.noreply.github.com>

routes/cookbook_helpers.py

@@ -552,6 +552,27 @@ def _append_llama_cpp_linux_accel_build_lines(runner_lines: list[str]) -> None:
     runner_lines.append('      cmake -B build -DCMAKE_BUILD_TYPE=Release && cmake --build build -j"$NPROC" --target llama-server && ln -sf ~/llama.cpp/build/bin/llama-server ~/bin/llama-server')
     runner_lines.append('    fi')
 
+
+def _llama_cpp_rebuild_cmd() -> str:
+    """Shell command that clears the Cookbook-managed llama.cpp build.
+
+    Removes the cached ``llama-server`` symlink and the ``~/llama.cpp/build``
+    directory so the next llama.cpp serve recompiles from source, picking up a
+    CUDA or HIP toolchain if one is now available. The serve bootstrap only
+    builds when ``llama-server`` is missing from PATH, so without this an
+    existing CPU-only build is reused forever. It deliberately installs and
+    downloads nothing; the rebuild itself happens on the next serve.
+    """
+    return (
+        'mkdir -p "$HOME/bin" && '
+        'rm -f "$HOME/bin/llama-server" && '
+        'rm -rf "$HOME/llama.cpp/build" && '
+        'echo "[odysseus] Cleared the cached llama.cpp build. '
+        'Re-launch the serve task to rebuild llama-server from source '
+        '(CUDA or HIP will be used if a toolchain is now available)."'
+    )
+
+
 class ModelDownloadRequest(BaseModel):
     repo_id: str
     include: str | None = None  # glob pattern e.g. "*Q4_K_M*"