feat(hero): responsive srcset for carousel slides

Phones were downloading the full 1.5 MB / 2 MB hero JPEG. Now the
image macro emits a `srcset` ladder (480/768/1200/1600/2560/3840/5345 w)
with `sizes="100vw"`, so a 480 px-wide phone picks the ~19 KB variant
and 5K screens still get the native resolution.

Also drop `loading="lazy"` on the first slide — it is the LCP element
and should load eager.

Macro changes:
- Add `srcset_widths` (CSV string, since Tera macros only allow scalar
  defaults) and `sizes` parameters.
- Add `lazy=true` parameter; callers can pass `lazy=false` for
  above-the-fold images.

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

templates/index.html

@@ -16,11 +16,18 @@
   <div class="carousel-inner" role="listbox">
     {% for slide in section.extra.slides %}
       <div class="item{% if loop.first %} active{% endif %}">
-        {# Serve the source image at full resolution — Bootstrap-3 only
-           constrains via `max-width: 100%`, so on viewports wider than
-           the natural image width the slide would otherwise sit short
-           of the column edge. Live does the same (no resize). #}
-        {{ image::render(src=slide.image, alt=slide.alt) }}
+        {# Bootstrap-3 only constrains hero images via `max-width: 100%`,
+           so the natural image width has to cover the widest viewport
+           we care about. We hand the browser a `srcset` ladder up to
+           5345 px (the source's native width) and let it pick — phones
+           grab ~480-800 px, desktops 1600-2560 px, 5K screens the full
+           file. The first slide is the LCP element, so it loads eager. #}
+        {{ image::render(
+             src=slide.image,
+             alt=slide.alt,
+             srcset_widths="480,768,1200,1600,2560,3840,5345",
+             sizes="100vw",
+             lazy=not loop.first) }}
         <div class="carousel-caption">
           <p>{{ slide.heading }}</p>
         </div>

templates/macros/image.html

@@ -1,22 +1,37 @@
 {# Image-rendering macro, used both by templates (directly via import)
    and indirectly by the `img` shortcode (which forwards to it).
 
-   With `width > 0`: resizes via Zola's image pipeline (file size benefit)
-   but emits no width/height attributes — the theme CSS sizes the image,
-   matching the live Grav site's pattern. Avoiding the HTML width attribute
-   sidesteps a stretch artefact triggered by Bootstrap-3's
-   `img { height: auto !important }` interacting with `.float-right`
-   padding.
+   Modes:
+   - `srcset_widths="480,768,…"` non-empty: emit a responsive `srcset`
+     over those widths. `sizes` should describe the layout (e.g.
+     "100vw" for the full-width hero). The `src` fallback uses the
+     largest width. CSV form because Tera macros only allow scalar
+     defaults, not arrays.
+   - `width > 0`: single resized variant via Zola's image pipeline.
+   - `width = 0` (default): raw <img> straight from /static/images/.
 
-   With `width = 0` (default): emits the raw <img> straight from
-   /static/images/ — same shape, no resize. #}
+   In all modes we omit the HTML width/height attributes — the theme CSS
+   sizes the image, matching the live Grav site's pattern. Avoiding the
+   HTML width attribute sidesteps a stretch artefact triggered by
+   Bootstrap-3's `img { height: auto !important }` interacting with
+   `.float-right` padding.
 
-{% macro render(src, width=0, alt="", class="") %}
+   `lazy=false` switches off `loading="lazy"`. Use it for above-the-fold
+   images (LCP candidates) like the first hero slide. #}
+
+{% macro render(src, width=0, alt="", class="", srcset_widths="", sizes="", lazy=true) %}
   {%- set width = width | int -%}
-  {%- if width > 0 -%}
+  {%- set loading_attr = "" -%}
+  {%- if lazy -%}{%- set loading_attr = ' loading="lazy"' -%}{%- endif -%}
+  {%- if srcset_widths -%}
+    {%- set widths = srcset_widths | split(pat=",") -%}
+    {%- set largest = widths | last | trim | int -%}
+    {%- set fallback = resize_image(path="static/images/" ~ src, width=largest, op="fit_width", quality=82) -%}
+    <img src="{{ fallback.url | safe }}" srcset="{%- for w_str in widths -%}{%- set w = w_str | trim | int -%}{%- set r = resize_image(path="static/images/" ~ src, width=w, op="fit_width", quality=82) -%}{{ r.url | safe }} {{ w }}w{%- if not loop.last -%}, {%- endif -%}{%- endfor -%}" sizes="{{ sizes }}" alt="{{ alt }}"{% if class %} class="{{ class }}"{% endif %}{{ loading_attr | safe }}>
+  {%- elif width > 0 -%}
     {%- set resized = resize_image(path="static/images/" ~ src, width=width, op="fit_width", quality=85) -%}
-    <img src="{{ resized.url | safe }}" alt="{{ alt }}"{% if class %} class="{{ class }}"{% endif %} loading="lazy">
+    <img src="{{ resized.url | safe }}" alt="{{ alt }}"{% if class %} class="{{ class }}"{% endif %}{{ loading_attr | safe }}>
   {%- else -%}
-    <img src="/images/{{ src | safe }}" alt="{{ alt }}"{% if class %} class="{{ class }}"{% endif %} loading="lazy">
+    <img src="/images/{{ src | safe }}" alt="{{ alt }}"{% if class %} class="{{ class }}"{% endif %}{{ loading_attr | safe }}>
   {%- endif -%}
 {% endmacro render %}