Add optional Alternative.image and Feature.color/icon for richer layouts

image: a banner url rendered as <img>, client-fetched directly (never
server-side, so none of ResourceSource::Url's SSRF concern applies).
color: any CSS color, set as the feature's own --feature-accent custom
property (never interpolated into a stylesheet) - draws a left-border
accent; unset means no border, not a fallback to the global --accent.
icon: an Iconify name (lucide:star etc.), rendered via Iconify's public
SVG API - no icon library bundled, matching this app's zero-JS-
dependency content otherwise keeps to.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/content.rs

@@ -62,6 +62,13 @@ pub struct Alternative {
     pub consequence: Vec<String>,
     #[serde(default)]
     pub encouragements: Vec<String>,
+    /// A banner image url, rendered above the description - purely
+    /// decorative, no upload/hosting mechanism of its own, just an
+    /// already-hosted url the browser fetches directly (unlike
+    /// `ResourceSource::Url`, this is never fetched server-side, so it
+    /// carries none of that variant's SSRF concern).
+    #[serde(default)]
+    pub image: Option<String>,
     #[serde(default)]
     pub features: Vec<Feature>,
     /// Names a NATS KV bucket to also durably store this submission
@@ -99,6 +106,23 @@ pub struct Feature {
     pub name: String,
     #[serde(default)]
     pub description: String,
+    /// Any valid CSS color (hex, named, rgb()/oklch()/...) - set as this
+    /// feature's own `--feature-accent` custom property rather than
+    /// interpolated into a stylesheet, so a bad value just fails to
+    /// apply instead of being live CSS content could inject arbitrary
+    /// rules into. Unset means no accent border at all (see
+    /// style/main.css's `.feature`), not a silent fallback to the
+    /// global `--accent` - a feature that never asked for a color
+    /// shouldn't suddenly gain a visible border.
+    #[serde(default)]
+    pub color: Option<String>,
+    /// An Iconify icon name (`{prefix}:{name}`, e.g. `lucide:star`),
+    /// rendered via Iconify's public SVG API
+    /// (`https://api.iconify.design/{icon}.svg`) - no icon library
+    /// bundled here, matching this app's zero-JS-dependency content
+    /// otherwise keeps to.
+    #[serde(default)]
+    pub icon: Option<String>,
     #[serde(default)]
     pub requirements: Vec<Requirement>,
     /// Live data this feature pulls in. Read-only unless `transitions`