feat: UI overhaul — accent color, Inter font, widget icons, diversified colors

- Add teal accent color scale and activate Inter font via @font-face
- Neutralize nav/footer hovers from primary blue to surface neutrals
- Apply accent color to hero subtitle, FAB, CTA buttons, card hovers
- Fix reply post-type color from generic primary to distinctive sky blue
- Create centralized icon macro (icon.njk) with 24 reusable SVG icons
- Add per-widget-type icons and colored left-accent borders to all sidebars
- Update .p-category tags from blue to neutral surface with border
- Diversify color vocabulary: red (likes), amber (bookmarks/blogroll),
  green (reposts), purple (funkwhale), sky (replies), orange (subscribe)

Confab-Link: http://localhost:8080/sessions/bd3f7012-c703-47e9-bfe2-2ad04ce1842d

_includes/components/webmentions.njk

@@ -194,7 +194,7 @@
     >
     <button
       type="submit"
-      class="px-4 py-2 text-sm font-medium text-white bg-primary-600 hover:bg-primary-700 rounded transition-colors">
+      class="px-4 py-2 text-sm font-medium text-white bg-accent-600 hover:bg-accent-700 rounded transition-colors">
       Send
     </button>
   </form>