fix(home): card fills the slide; preview uses photo's natural aspect

Two complaints, one root cause: the FrameCard was floating in the slide with
a min-height-padded preview, so (1) photos got top/bottom gray bars instead
of fitting their container, and (2) there was a fat empty gap between the
card body and the bottom nav.

Restructured the large card to flex-fill its slide:
- preview hugs the photo's intrinsic aspect ratio (img with width:100%
  height:auto); no min-height, no aspect-ratio override → no letterbox
- card body has flex:1, info pinned at top, Add Photo button pinned at
  bottom via margin-top:auto and width:100%
- HomeView main / single-card / carousel all flex:1 down through the
  layout so the slide gets the full available height
- empty-state placeholder still reserves the device's aspect so the
  card doesn't jump while images load

Result: the photo fills its container left/right with no bars; the body
absorbs all remaining space below, with the action button always sitting
just above the bottom nav.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/views/HomeView.vue

@@ -348,6 +348,7 @@ async function saveSettings() {
 
 <style scoped lang="scss">
 .home-view {
+  flex: 1;
   padding: var(--space-4) var(--space-4) calc(var(--bottom-nav-height) + var(--space-4));
   display: flex;
   flex-direction: column;
@@ -397,6 +398,7 @@ async function saveSettings() {
   }
 
   &__single {
+    flex: 1;
     display: flex;
     flex-direction: column;
   }
@@ -405,6 +407,8 @@ async function saveSettings() {
   // page's side padding so cards reach edge-to-edge; each slide pads itself
   // back in so the card visually aligns with the rest of the page.
   &__carousel {
+    flex: 1;
+    min-height: 0;
     display: flex;
     overflow-x: auto;
     overflow-y: hidden;
@@ -423,6 +427,8 @@ async function saveSettings() {
     padding: 0 var(--space-4);
     scroll-snap-align: start;
     scroll-snap-stop: always;
+    display: flex;
+    flex-direction: column;
   }
 
   &__dots {