feat: lightbox fill/fit toggle mode

Add fill state (default false) to the Alpine lightbox data:
- FILL button (top-right, left of ✕): switches media to object-cover,
  filling the entire overlay edge-to-edge; button turns heat pink
- FIT restores object-contain with padding; button returns to fog
- Keyboard shortcut F toggles fill while lightbox is open
- close() resets fill to false for the next open
- transition-all duration-300 on container padding and media for
  a smooth morph between modes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

layouts/posts/single.html

@@ -116,15 +116,17 @@
     x-data="{
       open: false,
       idx:  0,
+      fill: false,
       items: {{ $lbItems | jsonify }},
       show(i)  { this.idx = i; this.open = true },
-      close()  { this.open = false },
+      close()  { this.open = false; this.fill = false },
       prev()   { this.idx = (this.idx - 1 + this.items.length) % this.items.length },
       next()   { this.idx = (this.idx + 1) % this.items.length }
     }"
     @keydown.escape.window="open && close()"
     @keydown.arrow-left.window="open && prev()"
     @keydown.arrow-right.window="open && next()"
+    @keydown.f.window="open && (fill = !fill)"
   >
 
     <div class="flex items-center gap-5 mb-10">
@@ -174,6 +176,15 @@
         <div class="absolute top-5 left-1/2 -translate-x-1/2 label text-fog tabular-nums"
           x-text="`${String(idx + 1).padStart(2,'0')} / ${String(items.length).padStart(2,'0')}`"></div>
 
+        <!-- Fill / Fit toggle -->
+        <button
+          @click="fill = !fill"
+          class="absolute top-3 right-16 w-10 h-10 flex items-center justify-center label transition-colors"
+          :class="fill ? 'text-heat hover:text-chalk' : 'text-fog hover:text-heat'"
+          :aria-label="fill ? 'Switch to fit mode' : 'Switch to fill mode'"
+          x-text="fill ? 'FIT' : 'FILL'"
+        ></button>
+
         <!-- Close -->
         <button
           @click="close()"
@@ -190,14 +201,18 @@
         >{{ partial "icon.html" "arrow-left" }}</button>
 
         <!-- Media -->
-        <div class="w-full h-full flex items-center justify-center px-14 md:px-20 py-14">
+        <div
+          class="w-full h-full flex items-center justify-center transition-all duration-300"
+          :class="fill ? 'p-0' : 'px-14 md:px-20 py-14'"
+        >
           <!-- Video -->
           <video
             x-show="items[idx] && items[idx].video"
             :src="items[idx] && items[idx].video ? items[idx].video : ''"
             :poster="items[idx] ? items[idx].img : ''"
             x-effect="if (open && items[idx] && items[idx].video) { $el.load(); $el.play() }"
-            class="max-w-full max-h-full object-contain"
+            :class="fill ? 'w-full h-full object-cover' : 'max-w-full max-h-full object-contain'"
+            class="transition-all duration-300"
             controls loop muted playsinline
           ></video>
           <!-- Image -->
@@ -205,7 +220,8 @@
             x-show="items[idx] && !items[idx].video"
             :src="items[idx] && !items[idx].video ? items[idx].img : ''"
             alt=""
-            class="max-w-full max-h-full object-contain"
+            :class="fill ? 'w-full h-full object-cover' : 'max-w-full max-h-full object-contain'"
+            class="transition-all duration-300"
           >
         </div>