Let shortcuts be rebound from Settings

Every bind in keybinds.lua now goes through a small wrapper that
substitutes the chord from a stored override. Only the chord is taken
from settings; the action is always the Lua value written in that file,
so an override can move a shortcut but can never make one do something
else. That is the property that makes reading them from a file the user
can edit safe, and it is why the alternative -- storing dispatchers --
was not considered.

Overrides are keyed by the shipped chord rather than the description.
Keying by description moved every bind that shared one: rebinding
SUPER+C also moved the XF86Calculator hardware key onto the same chord,
silently costing it. Chords are unique; descriptions are not.

Applying needs hyprctl reload rather than a live hl.bind. Hyprland
reports Lua-defined binds with dispatcher "__lua" and a bytecode offset,
so the action cannot be reconstructed from outside to re-bind it; reload
re-runs the config, which re-reads the settings file.

The capture control ignores modifier-only presses, because every chord
passes through them and holding Super would otherwise be captured the
moment the modifier went down. It refuses a bare letter, which would
swallow ordinary typing, and refuses a key with no keysym name rather
than storing something that would fail to bind. Rebinding onto a chord
already in use is refused rather than shadowing the existing shortcut.

The refactor was verified by snapshotting all 113 binds before and
after: the keymap is byte-identical, and identical again after applying
an override and resetting it.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

config/dot/quickshell/modules/settings/qmldir

@@ -34,3 +34,4 @@ WallpaperPicker 1.0 WallpaperPicker.qml
 ApplicationsPage 1.0 ApplicationsPage.qml
 DockPinsEditor 1.0 DockPinsEditor.qml
 DockAppPicker 1.0 DockAppPicker.qml
+ShortcutCapture 1.0 ShortcutCapture.qml