feat(ui): ribbon collapse-mode selector + start-page recent controls

Ribbon:
- A ▾ selector at the right of the tab row picks the tool-panel density —
  Auto (size to window), Full, Compact, or Collapsed. The choice is persisted.
  It hides itself when the tool row is already tight, giving the cramped tab
  row its space back.

Start page:
- The Recent Documents card is restyled to match the Supporters card
  (rounded corners, 20px padding, matching title + colours).
- Its scrollbar no longer sits on top of the row ✕ remove buttons (a gutter
  is reserved).
- A footer control sets how many recent files to keep: [-] an editable count
  [+] with the max shown. Typing applies on Enter; an over-max entry snaps to
  the max. The limit is persisted and enforced on load and on every open.
- The "Support on Patreon" heart is now the SVG icon instead of a font glyph.

Config:
- Add a shared `config::config_dir()` helper; the recent-files and status-bar
  stores now go through it instead of each re-deriving the path. New minus.svg
  icon for the stepper.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/icons/ui/minus.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#000000" stroke-width="2.2" stroke-linecap="round"><path d="M5 12 L19 12"/></svg>