Add per-feed visibility settings panel

Visitors can hide individual feeds via a gear-icon panel in the header.
Preferences live in localStorage; the server ships the full source list
(name + channel) as window.PETE_SOURCES so the panel lists every feed,
grouped by channel, regardless of what's on the current page.

internal/web/templates/_card.html

@@ -1,5 +1,6 @@
 {{define "card"}}
 <a href="{{.Story.ArticleURL}}" target="_blank" rel="noopener noreferrer"
+   data-story-card data-source="{{.Story.Source}}" data-channel="{{.Story.Channel}}"
    class="group relative block rounded-3xl bg-[color:var(--card)] border-2 {{if .Story.Posted}}border-theme-{{.Theme}} glow-theme-{{.Theme}}{{else}}border-[color:var(--ink)]/10{{end}} shadow-pete overflow-hidden hover:-translate-y-1 hover:shadow-pete-lg transition">
   {{if .Story.ImageURL}}
   <div class="aspect-[16/10] w-full overflow-hidden bg-[color:var(--ink)]/5">