Add context listing and switching to the TUI

Press c to list cv4pve-cli contexts and switch between them. The
active context is always shown in the header.

- Switching runs `cv4pve-cli config use`, which changes the global
  current context. cv4pve-cli 2.4.0 has no per-call context option.
- `config list` has no JSON output, so its text lines are parsed.
- The guest table is cleared as soon as another context is chosen, so
  the old cluster's guests cannot be acted on.
- A generation counter drops guest listings that started before a
  switch, and g and c are refused while a switch runs. Without this,
  a reload already in flight could refill the table with the old
  cluster's guests, and VMIDs collide across clusters.

103 tests pass. The switch was tested live from mainframe to
mainframe-edge and back.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_cli.py

@@ -370,3 +370,52 @@ def test_status_reads_live_status_current(fake):
         "-o",
         "json",
     ]
+
+
+# --- contexts / use_context (spec: "Context switching" > cli.py) -------------
+
+CONTEXT_LIST = (
+    "* mainframe            mainframe.iot.home.arpa:8006   (api-token)\n"
+    "  mainframe-edge       mainframe-edge.home.arpa:8006   (api-token)\n"
+)
+
+
+def test_contexts_parses_current_name_host_auth(fake):
+    fake.result = (CONTEXT_LIST, "", 0)
+    assert cli.contexts() == [
+        {"name": "mainframe", "host": "mainframe.iot.home.arpa:8006",
+         "auth": "api-token", "current": True},
+        {"name": "mainframe-edge", "host": "mainframe-edge.home.arpa:8006",
+         "auth": "api-token", "current": False},
+    ]  # fmt: skip
+
+
+def test_contexts_runs_config_list(fake):
+    fake.result = (CONTEXT_LIST, "", 0)
+    cli.contexts()
+    assert fake.argv == ["cv4pve-cli", "config", "list"]
+
+
+def test_contexts_ignores_junk_and_empty_lines(fake):
+    fake.result = (
+        "\nContexts:\n" + CONTEXT_LIST + "\n   \nnot a context line\n",
+        "",
+        0,
+    )
+    assert [c["name"] for c in cli.contexts()] == ["mainframe", "mainframe-edge"]
+
+
+def test_contexts_empty_output_gives_empty_list(fake):
+    assert cli.contexts() == []
+
+
+def test_use_context_runs_config_use_with_name(fake):
+    cli.use_context("mainframe-edge")
+    assert fake.argv == ["cv4pve-cli", "config", "use", "mainframe-edge"]
+
+
+def test_use_context_unknown_name_raises_clierror(monkeypatch):
+    FakeRun(monkeypatch, "Error: Context 'x' not found.\n", returncode=3)
+    with pytest.raises(cli.CliError) as exc:
+        cli.use_context("x")
+    assert "Context 'x' not found" in str(exc.value)