feat: Use XDG_STATE_HOME for "log file" default

- Change: Change default value of "log file" config option to `"$XDG_STATE_HOME/termvisage/log"`.
- Change: Update "log file" documentation.
- Change: Update sample default config file.

Refs: #2

default_config.json

@@ -5,7 +5,7 @@
     "checkers": null,
     "getters": 4,
     "grid renderers": 1,
-    "log file": "~/.termvisage/termvisage.log",
+    "log file": "~/.local/state/termvisage/log",
     "max notifications": 2,
     "max pixels": 4194304,
     "multi": true,

docs/source/config.rst

@@ -104,8 +104,8 @@ These are top-level fields whose values control various settings of the viewer.
 .. confval:: log file
    :synopsis: The file to which logs are written.
    :type: string
-   :valid: An absolute path to a writable file
-   :default: ``"~/.termvisage/termvisage.log"``
+   :valid: A writable/creatable file path
+   :default: ``"{$XDG_STATE_HOME}/termvisage/log"``
 
    If the file:
 
@@ -116,9 +116,11 @@ These are top-level fields whose values control various settings of the viewer.
    Supports tilde expansion i.e a leading ``~`` (tilde) character is expanded to the
    current user's home directory.
 
+   The environment variable ``$XDG_STATE_HOME`` defaults to ``~/.local/state`` if undefined.
+
    .. warning::
       Relative paths are allowed but this will cause the log file to be written (or
-      created) relative to the **current working directory** every time the process
+      created) relative to the **current working directory** every time a session
       is started.
 
    .. note:: Overridden by :option:`-l`.

src/termvisage/config.py

@@ -577,7 +577,11 @@ def update_context_nav(
         "must be a non-negative integer",
     ),
     "log file": Option(
-        path.join("~", ".termvisage", "termvisage.log"),
+        path.join(
+            os.environ.get("XDG_STATE_HOME", path.join("~", ".local", "state")),
+            "termvisage",
+            "log",
+        ),
         lambda x: isinstance(x, str) and is_writable(x),
         "must be a string containing a writable/creatable file path",
     ),