Card image: paste button and site-icon (@favicon) option

The banner panel's card-image block gains a pasteboard button (an image
uploads; an image URL goes as the save message's image setting itself and
is fetched/stored server-side, cross-origin being CORS-blocked for the
browser) and a site-icon button saving the @favicon sentinel, which
resolves to Data.favicon at render time and follows favicon changes.

docs/editing.md

@@ -7,7 +7,7 @@ The Vue editor is a single tabbed `EditorShell.vue` mounted in a host div create
 The shell hosts five kept-alive tabs (ordered site-wide first — site, structure, localization — then, after a visual break, the per-page tabs — article, banner):
 
 - `PageEditor.vue` — CodeMirror + server-rendered preview over WebSocket `/_api/ws/editor`, previewing into the visible article; editor and article scrolls are linked piecewise-linearly, keyed on the section anchors' `data-line` (markdown source line the backend stamps on top-level anchored h1/h2s): the page follows the cursor (fractional, wrap-aware, scrolling only when the cursor's page position leaves the viewport, with an edge margin), the editor follows page scroll with a progress-based viewport anchor, applied instantly (the window keeps scrolling normally while any editor is open — the panel is fixed to the viewport's left edge, its top tracking the banner's bottom edge until the banner scrolls away — and the panel scrolls internally); anchored h2s carry their own edit pens that open the editor scrolled to that section; a format bar offers Markdown helpers — bold/italic/code/link/table/image upload (always block-level on a fresh blank-separated line of its own — a cursor on a non-empty line, e.g. inside an existing image tag, inserts after that line, never into it; always with an empty `""` caption, cursor inside the quotes), toggling fences (` ``` ` code blocks and `::: aside` containers share the same machinery: clicked inside one they remove it and select the content, otherwise they wrap the selection or the cursor's line, keeping it selected), and `.left`/`.right`/`.wide`/`.margin` placement toggles plus `.small`/`.large`/`.huge` text-size toggles (brace attributes on the block at the cursor, mutually exclusive within each group; on `:::` containers a placement class replaces the container name instead — `::: aside` → `::: margin`), with Ctrl/Cmd-B/I/S bindings — for the hard-to-remember syntax. Edits content and title only, never the path.
-- `BannerEditor.vue` — per-page banner HTML + banner design selector, previewed into `#page-banner`, plus the page's card image (`Node.image`, inherited by the subtree): just an upload button and a ✕ clearing the node's own (back to inherit) — the label states which image is in use (none / set for this article, “used in /<path>/*” when it has children / mined from the article / inherited from …) and the card previews below show it. Below it, the site's own cards preview in both modes (small and large) with the real `.card` markup and styles from pagerite.css — theme variables included, they are the site's look — scaled down via font-size (the card internals are all em, so the layout proportions match real cards exactly); both render the effective card image (the node's own, else the image the server mines from the article, else the inherited one), and the description appears only in the small format, like the backend's `_card`. The previews double as the card-mode selector for the per-article `Node.large` override: clicking one forces that mode (thin solid outline), clicking the selected one returns to automatic; under automatic the mode auto currently resolves to gets a dashed marker (both outlines — selection never shifts the layout), approximated from image presence only (the server's dimension probe is not available in the panel).
+- `BannerEditor.vue` — per-page banner HTML + banner design selector, previewed into `#page-banner`, plus the page's card image (`Node.image`, inherited by the subtree): upload and paste (pasteboard: an image uploads, an image URL is fetched and stored server-side via the `save` socket message) buttons, a site-icon button (saves `@favicon`, which resolves to the current site icon at render time, following favicon changes) and a ✕ clearing the node's own (back to inherit) — the label states which image is in use (none / set for this article, “used in /<path>/*” when it has children / the site icon / mined from the article / inherited from …) and the card previews below show it. Below it, the site's own cards preview in both modes (small and large) with the real `.card` markup and styles from pagerite.css — theme variables included, they are the site's look — scaled down via font-size (the card internals are all em, so the layout proportions match real cards exactly); both render the effective card image (the node's own, else the image the server mines from the article, else the inherited one), and the description appears only in the small format, like the backend's `_card`. The previews double as the card-mode selector for the per-article `Node.large` override: clicking one forces that mode (thin solid outline), clicking the selected one returns to automatic; under automatic the mode auto currently resolves to gets a dashed marker (both outlines — selection never shifts the layout), approximated from image presence only (the server's dimension probe is not available in the panel).
 - `SiteEditor.vue` — site brand + optional custom brand HTML with image/video upload + theme selector + page-transition selector + font picker + favicon upload — clicking the preview tile picks a new one — + site-wide custom CSS, CSS injected into `<head id="pagerite-user">`.
 - `StructureEditor.vue` — the vue-draggable structure tree with always-editable title/slug inputs per row and a per-row flag dropdown setting the page's primary language (`Node.language`, inherited by the subtree).
 - `LocalizationEditor.vue` — the site-wide translation settings: target languages as a flag grid (toggles, grouped in geographic rows; see docs/localization.md), the refresh-all-translations button, and the translator service WebSocket URL(s) to connect `scripts/translator.py` to.