admin: split the blog collection per content file

Sveltia/Decap i18n structures (single_file, multiple_files,
multiple_folders) can neither produce <slug>/<locale>.md nor carry a
shared non-localized meta file, so the folder is exposed as three
monolingual entry collections pinned to their file by the path
template: blog -> <slug>/meta.md, blog-fr -> <slug>/fr.md, blog-en ->
<slug>/en.md. app_id and PKCE untouched.

public/admin/config.yml

@@ -20,19 +20,63 @@ backend:
 media_folder: public/images/blog
 public_folder: /images/blog
 
+# Blog content model: each post is a folder `content/blog/<slug>/` holding
+# meta.md (shared metadata) + fr.md + en.md (one file per language, localized
+# title + body). Sveltia/Decap i18n cannot express that layout: their i18n
+# structures are single_file (all locales in <slug>.<ext>), multiple_files
+# (<slug>.<locale>.<ext>) and multiple_folders (<locale>/<slug>.<ext>) — none
+# produces <slug>/<locale>.md, and none has a notion of a shared,
+# non-localized meta file. So instead of the i18n machinery, the folder is
+# exposed as three monolingual entry collections sharing the same `folder`,
+# each pinned to its file by the `path` template (which also keeps unrelated
+# files out of each collection's list).
+#
+# Workflow: create a post under "Blog" (meta.md), then add its bodies under
+# "Blog FR" / "Blog EN" — when creating there, type the post's slug (the
+# folder name) so the files land in the same folder.
 collections:
   - name: blog
     label: Blog
     label_singular: Post
     folder: content/blog
+    path: "{{slug}}/meta"
     create: true
     editor:
       preview: false
     fields:
       - { label: Title, name: title, widget: string }
       - { label: Date, name: date, widget: datetime, format: 'YYYY-MM-DD', date_format: 'YYYY-MM-DD', time_format: false }
-      - { label: Slug, name: slug, widget: string, required: false, hint: 'Kebab-case; must match the filename (content/blog/<slug>.md)' }
       - { label: Description, name: description, widget: string, required: false }
       - { label: Tags, name: tags, widget: list, required: false, default: [] }
       - { label: Draft, name: draft, widget: boolean, default: false }
       - { label: Share token, name: share, widget: string, required: false, pattern: ['^[a-z0-9]{2,16}$', 'Lowercase letters and digits only'], hint: '6-char token for the unlisted /blog/d/<slug>-<token> link — node scripts/new-share-token.mjs' }
+
+  - name: blog-fr
+    label: Blog FR
+    label_singular: Post (FR)
+    folder: content/blog
+    path: "{{slug}}/fr"
+    create: true
+    editor:
+      preview: false
+    summary: "{{dirname}} — {{title}}"
+    fields:
+      - { label: Title, name: title, widget: string }
+      - { label: Description, name: description, widget: string, required: false }
+      - { label: Body, name: body, widget: markdown }
+    # `{{slug}}` on creation comes from the entry slug editor — type the post's
+    # folder name there; opening an existing file always resolves it correctly.
+
+  - name: blog-en
+    label: Blog EN
+    label_singular: Post (EN)
+    folder: content/blog
+    path: "{{slug}}/en"
+    create: true
+    editor:
+      preview: false
+    summary: "{{dirname}} — {{title}}"
+    fields:
+      - { label: Title, name: title, widget: string }
+      - { label: Description, name: description, widget: string, required: false }
+      - { label: Body, name: body, widget: markdown }