feat(flextable): add reusable Civilytics flextable branding helpers

Generalize the flextable brand styling and "export to PNG + stamp logo"
pattern repeated across Civilytics projects into two small functions:

- style_flextable_civilytics(): applies only the visual brand (header
  fill/color, body font/size, zebra striping guarded for <2 rows,
  borders, footer styling, fixed layout) to an already-structured
  flextable. Every value is an overridable parameter; zebra toggles
  striping. Structure (labels, headers, widths, alignment, footer text)
  stays with the caller.
- save_branded_flextable_png(): exports a styled flextable to PNG via
  ragg::agg_png() sized to the table plus extra_height headroom, then
  optionally stamps the logo via stamp_logo_png() (... forwarded).

flextable, officer, and ragg are added to Suggests (not Imports) to keep
the base install light; both functions reference them fully qualified and
guard with requireNamespace() + an install hint. Real round-trip tests
cover styling, 1-row idempotence, PNG export, and extra_height.

DESCRIPTION

@@ -31,7 +31,10 @@ Encoding: UTF-8
 Suggests:
     testthat (>= 3.0.0),
     tidycensus,
-    quarto
+    quarto,
+    flextable,
+    officer,
+    ragg
 Config/testthat/edition: 3
 Config/roxygen2/version: 8.0.0
 RoxygenNote: 7.3.3