feat(11-03): Alpine.js theme + i18n stores in base.html with top-right controls

- Add Alpine.store('theme') for Light/Dark/System cycling with localStorage persistence
- Add Alpine.store('i18n') with FR/EN translations for all static UI strings
- Add topbar with theme toggle button (:aria-label, @click cycle) and lang button
- Wrap main content in .main-wrapper + .topbar for layout structure
- Add .main-wrapper, .topbar, .topbar-controls styles to app.css
- Add test_theme_toggle_present integration test
- Fix test_dashboard_shows_recent_printers assertion (string now in i18n JS too)

imptune/static/app.css

@@ -91,6 +91,36 @@ nav.sidebar li {
   border-left-color: var(--pico-primary, #1a73e8);
 }
 
+/* Main wrapper: fills remaining space beside sidebar, stacks topbar + content */
+.main-wrapper {
+  flex: 1 1 auto;
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+}
+
+/* Topbar: holds top-right controls */
+.topbar {
+  display: flex;
+  justify-content: flex-end;
+  align-items: center;
+  padding: 0.5rem 1rem;
+  border-bottom: 1px solid var(--pico-muted-border-color, #e0e0e0);
+  gap: 0.5rem;
+}
+
+.topbar-controls {
+  display: flex;
+  gap: 0.5rem;
+  align-items: center;
+}
+
+.topbar-controls button {
+  padding: 0.3rem 0.6rem;
+  font-size: 0.85rem;
+  min-width: 2.2rem;
+}
+
 /* Main content */
 .main-content {
   flex: 1 1 auto;