Consolidate omarchy-shell Hyprland rules; float settings panel

Rename apps/quickshell.lua to apps/omarchy-shell.lua and gather every
rule that targets a Quickshell surface owned by the shell:

  - menu and image-selector layer rules (image-selector moved out of
    apps/system.lua, where it didn't belong with generic floating /
    media / idle rules)
  - new window rules so the settings panel floats centered at 960x700
    instead of tiling

The settings panel is a transient dialog, not a workspace surface, so
tiling it disrupts the layout and leaves it awkwardly tall when it lands
in a half-screen split. Class+title scoping keeps the rule from leaking
into other org.quickshell windows (menu, image picker, etc.).

default/hypr/apps/system.lua

@@ -21,6 +21,3 @@ hl.window_rule({ match = { tag = "pop" }, rounding = 8 })
 
 -- Prevent idle while open.
 hl.window_rule({ match = { tag = "noidle" }, idle_inhibit = "always" })
-
--- Image selector.
-hl.layer_rule({ match = { namespace = "omarchy-image-selector" }, no_anim = true, animation = "none" })