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>

config/fcitx5/config

@@ -9,7 +9,9 @@
 [Hotkey/DeactivateKeys]
 0=Muhenkan
 
+# A lone Shift leaves an engine like Pinyin for Latin input and returns to it.
 [Hotkey/AltTriggerKeys]
+0=Shift_L
 
 [Hotkey/EnumerateGroupForwardKeys]