Stop server-provided values from reaching a shell or escaping /dsc/seafile

Commands were joined into a string and passed to "su -c", so a login shell
parsed every interpolated value. A library name is chosen on the server, and
"sync -d /dsc/seafile/<name>" put it straight into that string: a library
named 'Docs; touch /tmp/pwned; #' ran as an arbitrary command. Confirmed
against the built image before and after this change.

Every command is now an argument list executed by "runuser --", which execs
directly with no shell. runuser -u sets the same HOME and a PATH that still
finds the seaf-cli wrapper, so the daemon environment is unchanged.

The password no longer needs its surrounding quotes, which also fixes
passwords containing quotes, dollar signs, and backticks.

Library names are untrusted, so dsc/paths.py reduces each one to a single
path component and proves the result stays under the libraries directory
with symlinks resolved. Empty, ".", "..", and names containing a separator
are skipped with a warning; names that would collide are suffixed with their
library ID rather than sharing a directory.

Tests cover quotes, command substitutions, separators, newlines, absolute
paths, parent traversal, Unicode, and colliding names. They run in a separate
CI job so application failures stay distinguishable from packaging failures.

tests/conftest.py

@@ -0,0 +1,21 @@
+"""Make ``dsc.client`` importable outside the container.
+
+``seafile`` and ``pysearpc`` ship with the Seafile CLI AppImage and only exist
+inside the image. The application tests must not need a live server or that
+AppImage, so a stub module stands in for the RPC client.
+"""
+
+import sys
+import types
+from unittest import mock
+
+
+def _install_seafile_stub():
+    if "seafile" in sys.modules:
+        return
+    seafile = types.ModuleType("seafile")
+    seafile.RpcClient = mock.MagicMock(name="RpcClient")
+    sys.modules["seafile"] = seafile
+
+
+_install_seafile_stub()