feat(kanban): choose a colour per column

"Spalten anpassen" gets a colour dot per column that opens the choice: grey, anthracite (theme
text colour), green, yellow, blue, red, purple or the accent. columnDotClass reads the column's
colour (server defaults emerald/amber count as green/yellow; columns without one keep the colour
of their role) and drives the Kanban header, the status dot in the list view and the task detail.
Blue and purple come from new theme variables --color-info / --color-purple with fallbacks for
light and dark themes. Docs and changelog updated; v0.31.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/index.css

@@ -431,6 +431,17 @@ body {
     --code-variable: #9cdcfe;
   }
 
+  /* Extra tones (Kanban column colours). Fallbacks like the code tokens: a theme's own values win. */
+  :where(:root) {
+    --color-info: #60a5fa;
+    --color-purple: #a78bfa;
+  }
+
+  :where(:root:not(.dark)) {
+    --color-info: #2563eb;
+    --color-purple: #7c3aed;
+  }
+
   :where(:root:not(.dark)) {
     --code-keyword: #0000ff;
     --code-string: #a31515;