fix(config): use / as default cancelled icon

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.

doc/pending.txt

@@ -982,7 +982,7 @@ Fields: ~
                            {priority}  Priority task character. Default: '!'
                            {wip}       Work-in-progress character. Default: 'w'
                            {blocked}   Blocked task character. Default: 'b'
-                           {cancelled} Cancelled task character. Default: 'c'
+                           {cancelled} Cancelled task character. Default: '/'
                            {due}       Due date prefix. Default: '.'
                            {recur}     Recurrence prefix. Default: '~'
                            {category}  Category prefix. Default: '#'

lua/pending/config.lua

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