Replace swaylock with gtklock; centralise theme palette

- Replace swaylock with gtklock (username + password form lockscreen)
- Add gtklock/config.ini and gtklock/style.css (Tomorrow Night theme)
- Update niri keybindings and swayidle to invoke gtklock -d
- Update packages.txt: swaylock -> gtklock
- Update install.sh: add AUR helper detection (yay/paru), symlink gtklock configs
- Add theme/colors.css (CSS custom properties) and theme/colors.json
- Update waybar/style.css, wofi/style.css, gtklock/style.css to import shared palette
- Update README.md with project summary and component table
- Add ARCHITECTURE.md and CLAUDE.md

theme/colors.css

@@ -0,0 +1,22 @@
+/* Tomorrow Night - shared color palette */
+/* @import this file from waybar, wofi, gtklock CSS */
+
+* {
+  --tn-bg:        #1d1f21;
+  --tn-bg-alt:    #282a2e;
+  --tn-bg-high:   #373b41;
+  --tn-fg:        #c5c8c6;
+  --tn-fg-dim:    #969896;
+  --tn-fg-muted:  #707880;
+  --tn-blue:      #81a2be;
+  --tn-green:     #b5bd68;
+  --tn-yellow:    #f0c674;
+  --tn-red:       #cc6666;
+  --tn-magenta:   #b294bb;
+  --tn-cyan:      #8abeb7;
+
+  /* Alpha variants */
+  --tn-bg-a90:    rgba(29, 31, 33, 0.90);
+  --tn-bg-a95:    rgba(29, 31, 33, 0.95);
+  --tn-bg-a96:    rgba(29, 31, 33, 0.96);
+}

theme/colors.json

@@ -0,0 +1,15 @@
+{
+  "name": "Tomorrow Night",
+  "background":    "#1d1f21",
+  "background_alt": "#282a2e",
+  "background_high": "#373b41",
+  "foreground":    "#c5c8c6",
+  "foreground_dim": "#969896",
+  "foreground_muted": "#707880",
+  "blue":          "#81a2be",
+  "green":         "#b5bd68",
+  "yellow":        "#f0c674",
+  "red":           "#cc6666",
+  "magenta":       "#b294bb",
+  "cyan":          "#8abeb7"
+}