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/tailwind.config.js

@@ -59,6 +59,11 @@ export default {
           info: v('--accent-hover'),
           danger: v('--color-danger'),
         },
+        // Extra tones for Kanban columns; fallbacks in index.css, themes may set their own
+        tone: {
+          blue: v('--color-info'),
+          purple: v('--color-purple'),
+        },
         border: v('--border-color'),
         // Aliases for names used in components (e.g. avatar initials), mapped to theme colours
         primary: v('--accent-primary'),