Add a favicon and rework the Design page

Blogs had no icon at all, and browsers probing /favicon.ico ended up
rendering the 404 page. Ship a default paper & ink icon (svg + png) and
let each blog replace it from the Design page; /favicon.ico now serves
the default or redirects to the blog's own image. ICO uploads accepted.

The Design page gains colour schemes, reset to defaults, thumbnail
image pickers, jump links and a sticky save bar, plus new options:
heading font, link underline/hover, content box style and padding,
header image height, post dates and format, footer RSS toggle, menu
alignment/style/hover and sidebar width. The Menu card picks the
position with illustrated tiles and shows only the options that apply
to it, using CSS :checked rules rather than JavaScript.

All new defaults reproduce the previous look, so existing blogs are
unchanged until edited. Theme is jsonb, so no migration.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sd8UPWrvyYCLj97JexNw3A

1 files changed, 20 insertions, 2 deletions

diff --git a/AGENTS.md b/AGENTS.md
index 2352b85..b552615 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -86,9 +86,27 @@ internal/web/         server.go (host router, middleware, render helpers)
   value to an allowlist (hex colours, enum strings, uuid image ids) — this
   is what makes inlining the generated CSS safe. Add new options in the
   struct, `normalize`, `ThemeFromForm`, the CSS template and
-  `dashboard/design.html`.
+  `dashboard/design.html`; defaults must reproduce the look blogs had
+  before the option existed. Non-CSS options (`ShowDates`, `DateFormat` via
+  `FormatDate`, `FooterShowRSS`, `Favicon`) are read by the blog templates.
+  Optional colours (`LinkHover`, `NavHover`) are `""` = inherit and come
+  from a colour input paired with a `<name>_custom` checkbox. Image fields
+  use the `imagepick` partial's radios: `none` clears, a uuid selects, no
+  value keeps (`pickImage`); an upload in `<name>_file` wins.
+  Options that only apply to one menu layout (`NavAlign` for horizontal
+  menus, `NavSidebarWidth` for the sidebar) are shown/hidden in
+  `design.html` with CSS only (`#np-*:checked ~ .only-*` in dashboard.css,
+  same trick for the `_custom` hover checkboxes) — no JS, and browsers
+  without `:checked` just show everything.
+  `Presets()`/`WithPreset` are the colour schemes (`POST /b/{sub}/design/preset`,
+  colours only); `POST /b/{sub}/design/reset` stores `DefaultTheme()`.
+- **Favicon**: `static/favicon.svg` + `favicon.png` are the defaults, linked
+  from both layouts. A blog can set `theme.favicon` (an image id) which the
+  blog layout links instead. `GET /favicon.ico` on every host
+  (`handleFavicon`) serves the png or redirects to the blog's image, so
+  icon-probing browsers never hit the 404 page; `favicon.ico` is reserved.
 - **Images**: `/media/{uuid}` served on every host with immutable cache
-  headers. Uploads are content-sniffed (png/jpeg/gif/webp). Deleting an
+  headers. Uploads are content-sniffed (png/jpeg/gif/webp/ico). Deleting an
   image clears theme references to it. `POST /b/{sub}/images/upload`
   answers JSON (`{id, filename, markdown}` / `{error}`) when the request
   has `Accept: application/json`; that is what the editor script calls.