feat: add screenshot gallery, theme selector, and 4 new curated themes

- Add comprehensive screenshot gallery in README.md and docs/GALLERY.md
  showcasing all desktop features, menus, and control panels
- Clean and sanitize screenshots (no private work projects or processes)
- Add 4 curated themes with accurate color palettes and btop themes:
  * Pirna (Bernardo Bellotto classic art)
  * Nordic Frost (Arctic aurora and mountain cliff)
  * Monolith (Sci-fi alien coast and bioluminescent cyan)
  * Dragon Fire (Dark fantasy gothic siege and fiery coral embers)
- Enhance theme selector in rice-theme and rice-menu with live grid thumbnails
- Add super + shift + t shortcut for instant visual theme switching
- Make wallpaper lookup robust across repo and user directories

themes/pirna/btop.theme

@@ -0,0 +1,53 @@
+# btop - Pirna, sampled from Bernardo Bellotto's Pirna from Sonnenstein Fortress
+theme[main_bg]=""
+theme[main_fg]="#ede8dc"
+theme[title]="#ede8dc"
+theme[hi_fg]="#78a6ba"
+theme[selected_bg]="#333d40"
+theme[selected_fg]="#78a6ba"
+theme[inactive_fg]="#828c8a"
+theme[graph_text]="#cdc7b7"
+theme[meter_bg]="#1f2527"
+theme[proc_misc]="#6e9e98"
+
+theme[cpu_box]="#445053"
+theme[mem_box]="#445053"
+theme[net_box]="#445053"
+theme[proc_box]="#445053"
+theme[div_line]="#445053"
+
+theme[temp_start]="#7b9456"
+theme[temp_mid]="#cca056"
+theme[temp_end]="#b8543f"
+
+theme[cpu_start]="#658ea4"
+theme[cpu_mid]="#78a6ba"
+theme[cpu_end]="#ba94a8"
+
+theme[free_start]="#465856"
+theme[free_mid]="#6e9e98"
+theme[free_end]="#84b5ae"
+
+theme[cached_start]="#4f6e82"
+theme[cached_mid]="#658ea4"
+theme[cached_end]="#78a6ba"
+
+theme[available_start]="#7a6238"
+theme[available_mid]="#cca056"
+theme[available_end]="#dfb56c"
+
+theme[used_start]="#74372a"
+theme[used_mid]="#b8543f"
+theme[used_end]="#c45842"
+
+theme[download_start]="#466275"
+theme[download_mid]="#658ea4"
+theme[download_end]="#78a6ba"
+
+theme[upload_start]="#695160"
+theme[upload_mid]="#a17b8f"
+theme[upload_end]="#ba94a8"
+
+theme[process_start]="#7b9456"
+theme[process_mid]="#8ea559"
+theme[process_end]="#cca056"

themes/pirna/colors.toml

@@ -0,0 +1,59 @@
+# Pirna - sampled from wallpapers/bellotto-pirna.jpg (Bernardo Bellotto, 1750s).
+# Every surface (bspwm, polybar, rofi, dunst, eww, st, kitty, GTK, btop,
+# xsecurelock, starship) is rendered from these keys by `rice-theme`.
+# Colour values must be #rrggbb; templates add alpha where they need it.
+
+name = "Pirna"
+mode = "dark"
+wallpaper = "bellotto-pirna.jpg"
+
+# --- surfaces, darkest to lightest ---
+bg        = "#121516"
+bg_alt    = "#181d1e"
+surface   = "#212729"
+overlay   = "#2b3336"
+selection = "#333d40"
+border    = "#445053"
+
+# --- text ---
+fg     = "#ede8dc"
+fg_alt = "#cdc7b7"
+dim    = "#828c8a"
+muted  = "#3c4648"
+
+# --- roles ---
+accent     = "#78a6ba"
+accent_alt = "#547e8f"
+urgent     = "#c45842"
+
+# --- named colours (ANSI 1-6) and their bright twins (9-14) ---
+red     = "#b8543f"
+green   = "#7b9456"
+yellow  = "#cca056"
+blue    = "#658ea4"
+magenta = "#a17b8f"
+cyan    = "#6e9e98"
+
+bright_red     = "#c45842"
+bright_green   = "#8ea559"
+bright_yellow  = "#dfb56c"
+bright_blue    = "#78a6ba"
+bright_magenta = "#ba94a8"
+bright_cyan    = "#84b5ae"
+
+# --- translucency: bar, popups, menus, notifications and terminals all use it ---
+alpha     = "0.78"
+alpha_hex = "c7"
+
+# --- shape and type, shared by every popup, menu and toast ---
+font          = "JetBrainsMono Nerd Font"
+font_size     = "11"
+radius        = "12"
+radius_small  = "8"
+border_width  = "1"
+window_border = "2"
+gap           = "10"
+icon_theme    = "Papirus-Dark"
+gtk_theme     = "Adwaita-dark"
+cursor_theme  = "Bibata-Modern-Classic"
+cursor_size   = "20"