Add Tactical sheet theme (new default): modern dark redesign

- New characterSheetStyle option 'tactical' (default), css/dg2-tactical.css.
- Dark federal-dossier design: charcoal surfaces, olive accent, Public
  Sans throughout, uppercase letterspaced name plate and section headers.
- Color-coded HP (olive) / WP (steel) / SAN (red) resource cards.
- Modern underline tab navigation; skill rows as hoverable panels with
  dimmed untrained skills and olive value badges; restyled item lists,
  inputs, and buttons.
- Item sheets (AppV1) carry the theme class and get matching dark styling.
- Fix: CSS selectors still targeted .deltagreen after the system-id
  rename, leaving most stock sheet styling dead - renamed to .deltagreen2.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

system.json

@@ -27,6 +27,7 @@
     ],
     "styles": [
         "css/deltagreen.css",
+        "css/dg2-tactical.css",
         "css/dialogue.css"
     ],
     "languages": [