fix(config): use / as default cancelled icon (#159)

Problem: the cancelled icon defaulted to `c`, inconsistent with the
`g/` keymap. Other statuses match: `gw` → `[w]`, `gb` → `[b]`.

Solution: change `icons.cancelled` default from `c` to `/` so the
keymap and state char are consistent.

lua/pending/config.lua

@@ -195,7 +195,7 @@ local defaults = {
     priority = '!',
     wip = 'w',
     blocked = 'b',
-    cancelled = 'c',
+    cancelled = '/',
     due = '.',
     recur = '~',
     category = '#',