feat: mobile UX fixes + Phase C one-click generation

Mobile UX:
- index.css: comprehensive mobile media queries — headings scale down,
  touch targets enforced, grid-2col-mobile collapse class, tablet breakpoint
- ContentArchive/ContentPage: grid minmax uses min(100%, Npx) to prevent
  overflow on small screens
- CharacterForm: visual style + rules editor grids collapse on mobile
- Dashboard: stat cards grid mobile-safe
- Layout: better nav touch targets, footer responsive gap

Phase C — One-Click Generation:
- Backend: GET /api/content/suggestions endpoint — LLM generates 3 topic
  ideas based on character profile and avoids repeating recent posts
- Dashboard: "Suggerimenti per oggi" section loads suggestions on mount,
  each card links to /content with prefilled topic + character
- ContentPage: reads ?topic= and ?character= URL params, auto-fills form
  and auto-triggers generation (one-click flow from Dashboard)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/ContentArchive.jsx

@@ -127,7 +127,7 @@ export default function ContentArchive() {
           </p>
         </div>
       ) : (
-        <div style={{ display: 'grid', gridTemplateColumns: 'repeat(auto-fill, minmax(380px, 1fr))', gap: '1rem' }}>
+        <div style={{ display: 'grid', gridTemplateColumns: 'repeat(auto-fill, minmax(min(100%, 380px), 1fr))', gap: '1rem' }}>
           {groups.map(group => {
             const activeIdx = activePlatform[group.key] || 0
             const activePost = group.posts[activeIdx] || group.posts[0]