fix: declare collection dependencies for incremental builds

Add eleventyImport.collections to templates that access collections
via Nunjucks loops (not pagination frontmatter). Without this,
Eleventy's --incremental mode skips these pages when content changes.

- index.njk: posts, feed (homepage recent posts widget)
- feed.njk: feed (RSS must update with new posts)
- feed-json.njk: feed (JSON feed must update with new posts)
- slashes.njk: pages (slash pages index)
- categories-index.njk: categories (categories index)

categories-index.njk

@@ -3,6 +3,9 @@ layout: layouts/base.njk
 title: Categories
 withSidebar: true
 permalink: categories/
+eleventyImport:
+  collections:
+    - categories
 ---
 <div>
   <h1 class="text-2xl sm:text-3xl font-bold text-surface-900 dark:text-surface-100 mb-2">Categories</h1>

feed-json.njk

@@ -1,6 +1,9 @@
 ---
 permalink: /feed.json
 eleventyExcludeFromCollections: true
+eleventyImport:
+  collections:
+    - feed
 ---
 {
   "version": "https://jsonfeed.org/version/1.1",

feed.njk

@@ -1,6 +1,9 @@
 ---
 permalink: /feed.xml
 eleventyExcludeFromCollections: true
+eleventyImport:
+  collections:
+    - feed
 ---
 <?xml version="1.0" encoding="utf-8"?>
 <rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom" xmlns:media="http://search.yahoo.com/mrss/">

index.njk

@@ -1,4 +1,8 @@
 ---
 layout: layouts/home.njk
 title: Home
+eleventyImport:
+  collections:
+    - posts
+    - feed
 ---

slashes.njk

@@ -3,6 +3,9 @@ layout: layouts/base.njk
 title: Slash Pages
 withSidebar: true
 permalink: /slashes/
+eleventyImport:
+  collections:
+    - pages
 ---
 <div class="h-feed">
   <h1 class="text-2xl sm:text-3xl font-bold text-surface-900 dark:text-surface-100 mb-2">Slash Pages</h1>