feat: add keybind cheatsheet plugin (#64)

* feat: add keybind cheatsheet

* docs: refresh keybind cheatsheet preview

* docs: use official generated keybind thumbnail

* feat(keybind-cheatsheet): preload cached snapshots

Move parser and cache ownership into an event-driven data service so the panel opens from a prepared last-known-good snapshot. Document the plugin's inspiration and independent v5 implementation.

keybind-cheatsheet/tests/fixtures/mango/main.conf

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+# Applications
+bind=SUPER,T,spawn,foot #"Terminal"
+bind=SUPER,B,spawn,firefox #"Browser"
+
+source=./nested.conf
+source-optional=./missing.conf
+source=./parts/*.conf
+
+# Media
+bind=NONE,XF86AudioRaiseVolume,spawn,wpctl set-volume @DEFAULT_AUDIO_SINK@ 5%+
+axisbind=SUPER,UP,viewtoleft_have_client
+mousebind=SUPER,btn_left,moveresize,curmove #"Move window"
+gesturebind=NONE,left,3,focusdir,left #"Focus left"
+switchbind=fold,spawn,swaylock #"Lock on close"

keybind-cheatsheet/tests/fixtures/mango/nested.conf

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+# Window Management
+bind=SUPER,Q,killclient #"Close window"
+source=./main.conf

keybind-cheatsheet/tests/fixtures/mango/parts/workspaces.conf

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+# Workspaces
+bind=SUPER,1,view,1 #"Workspace 1"