Switch out of an input method and back with a lone Shift (#14975)

* Switch out of an input method and back with a lone Shift

Fcitx's alternate trigger only acts while an engine like Pinyin is or was
active, so Shift + letter and plain Latin typing are unaffected. It needs
Shift releases intact, which the both-Shifts Caps Lock option prevented.

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

* Follow XDG_CONFIG_HOME in the Shift migration and test it

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

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/shell.d/input-method-checks.py

@@ -66,6 +66,7 @@ class InputMethodTest(unittest.TestCase):
         self.assertEqual(keys["Behavior"]["ActiveByDefault"], "False")
         self.assertNotIn("Control+space", keys["Hotkey/TriggerKeys"].values())
         self.assertEqual(set(keys["Hotkey/TriggerKeys"].values()), {"Zenkaku_Hankaku", "Hangul"})
+        self.assertEqual(keys["Hotkey/AltTriggerKeys"], {"0": "Shift_L"})
 
   def test_vconsole_variants_quotes_and_japanese_without_preference(self):
     self.preference.unlink()