feat(frontend): add routing, layout, i18n wiring, and restyle the conversion tool

Migrates the existing upload/format-select/quality/convert logic from
the old single-file App.jsx into a router shell (/ -> /fr//en/) with a
shared Layout (header, language switcher, dark-mode toggle, footer).
Tool logic itself is unchanged, only restyled into cards.

Part of the Ombrora Convert redesign (Task 2/5).

frontend/src/styles/layout.css

@@ -0,0 +1,50 @@
+.site-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 1rem 1.5rem;
+  border-bottom: 1px solid var(--color-border);
+}
+
+.brand {
+  font-weight: 700;
+  font-size: 1.25rem;
+  text-decoration: none;
+  color: var(--color-foreground);
+}
+
+.site-header-controls {
+  display: flex;
+  align-items: center;
+  gap: 1rem;
+}
+
+.language-switcher {
+  text-decoration: none;
+  font-weight: 500;
+  min-height: 44px;
+  display: inline-flex;
+  align-items: center;
+  padding: 0 0.5rem;
+}
+
+.theme-toggle {
+  background: none;
+  border: 1px solid var(--color-border);
+  border-radius: 8px;
+  width: 44px;
+  height: 44px;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  color: var(--color-foreground);
+}
+
+.site-footer {
+  margin-top: auto;
+  padding: 1.5rem;
+  text-align: center;
+  color: var(--color-foreground);
+  border-top: 1px solid var(--color-border);
+  font-size: 0.875rem;
+}