Consolidate omarchy-shell colors into a single shell.toml surface

Five plugins each owned their own colors.toml watcher with subtly
different parsers, plus two ad-hoc per-theme override files
(notifications.json, image-picker-colors.json). This collapses all of
that into one source of truth: Commons/Color.qml watches colors.toml +
shell.toml and exposes Color.bar.*, Color.popups.*, Color.notifications.*,
Color.menu.*, Color.imagePicker.* for every surface to bind to.

shell.toml is generated by the existing template pipeline from
default/themed/shell.toml.tpl. Themes can ship their own shell.toml to
override individual keys; everything missing falls back to the
foundational palette via root.pick(). Settings panel is intentionally
not themable beyond the foundational tokens.

last-horizon and solitude ship a minimal shell.toml to preserve their
historical 'notification border matches Hyprland active border'
behavior, which previously came from parsing the theme's hyprland.conf
(now removed).

default/themed/shell.toml.tpl

@@ -0,0 +1,50 @@
+# Omarchy shell colors. Defaults below derive from colors.toml. Any theme
+# can ship its own shell.toml at themes/<name>/shell.toml to override
+# individual keys — anything left out falls back to the values here.
+
+[bar]
+# The bar strip itself
+background = "{{ background }}"
+# Clock, weather, workspace numbers, indicator glyphs
+text       = "{{ foreground }}"
+# Module in an "active" state: screen recording, voxtype listening,
+# weather alert, update available. Same color is used everywhere a
+# module wants to call attention to itself.
+active     = "{{ color1 }}"
+
+[popups]
+# Body fill for every flyout opened from the bar: wifi, bluetooth, audio,
+# calendar, weather, control-center, notification-center, media. Body text
+# inside the flyouts is not separately themable — it follows [bar].text.
+background = "{{ background }}"
+# 1px outline around the flyout
+border     = "{{ foreground }}"
+
+[notifications]
+# Toast card body
+background = "{{ background }}"
+# Title and message text
+text       = "{{ foreground }}"
+# Card outline. Most themes match this to their Hyprland active-window
+# border so notifications visually belong to the focused window.
+border     = "{{ accent }}"
+# Bottom countdown bar that drains while the toast is on screen
+countdown  = "{{ accent }}"
+
+[menu]
+# Omarchy menu surface (the launcher-style picker invoked by the menu key)
+background = "{{ background }}"
+# Unhighlighted menu items
+text       = "{{ foreground }}"
+# Currently hovered / keyboard-selected item
+selected   = "{{ accent }}"
+
+[image-picker]
+# Backdrop scrim behind the picker (the picker draws this at ~70% alpha)
+background        = "{{ background }}"
+# Tile labels (image filename / theme name)
+text              = "{{ foreground }}"
+# 3px stroke around the currently selected tile
+selected-border   = "{{ accent }}"
+# 1px stroke around every other tile (picker applies ~28% alpha)
+unselected-border = "{{ foreground }}"