refactor(icons): unify category/header icon and use checkbox overlays

Problem: `header` and `category` were separate icons for the same
concept. The icon overlay replaced `[ ]` with a bare character,
hiding the markdown checkbox syntax. Header format `## ` produced
a double-space with single-char icons.

Solution: merge `header` into `category` (one icon for both header
lines and EOL labels). Overlay renders `[icon]` preserving bracket
syntax. Change header line format from `## ` to `# ` so the
2-char overlay (`# `) maps cleanly.

lua/pending/views.lua

@@ -133,7 +133,7 @@ function M.category_view(tasks)
       table.insert(lines, '')
       table.insert(meta, { type = 'blank' })
     end
-    table.insert(lines, '## ' .. cat)
+    table.insert(lines, '# ' .. cat)
     table.insert(meta, { type = 'header', category = cat })
 
     local all = {}