Add theme system: local themes, picker, static/dynamic mode

- blob_theme: apply local or shared themes, --dynamic recolors from
  the current wallpaper without changing it, --print saves the active
  theme's colors.toml (e.g. for a dynamic palette worth keeping)
- themes/: drop-in colors.toml files that install.sh syncs into
  Omarchy's theme dir, alongside any already-installed themes
- ThemePicker widget, walker menu, and Quick Settings tile mirroring
  the existing wallpaper picker, with color-swatch previews
- blob_wallpaper no longer forces dynamic mode on every wallpaper
  change; it only recolors if you're already in dynamic mode
- install.sh: fix a new-directory mkdir bug in backup_and_copy, and
  stop backing up wallpapers/themes (.bak dirs were showing up as
  fake entries in the picker) since both are pure git mirrors

Claude-Session: https://claude.ai/code/session_011kf23kB9NsaczSYazZsWs6

themes/flats/colors.toml

@@ -0,0 +1,23 @@
+accent = "#7babe3"
+cursor = "#cbd2d9"
+foreground = "#cbd2d9"
+background = "#0f1012"
+selection_foreground = "#0f1012"
+selection_background = "#7babe3"
+
+color0 = "#000000"
+color1 = "#d53838"
+color2 = "#38d4d5"
+color3 = "#d58638"
+color4 = "#d53886"
+color5 = "#38d586"
+color6 = "#3886d5"
+color7 = "#d4d4d4"
+color8 = "#949494"
+color9 = "#d53838"
+color10 = "#38d4d5"
+color11 = "#d58638"
+color12 = "#d53886"
+color13 = "#38d586"
+color14 = "#3886d5"
+color15 = "#d4d4d4"