feat(files): a file that is not media opens in a viewer — an image, a PDF, text as text

- GET /api/files/{id}/preview: an allow-list on the stored extension (downloads/preview.py) picks
  image / PDF / text; served inline under `CSP: sandbox; frame-ancestors 'self'`, the type from the
  table; HTML and SVG arrive as plain text; a text is its first 1 MiB (X-Preview-Truncated), sent
  as UTF-8 whatever it was written in (BOM, UTF-8, else cp1250); a NUL means binary: 415
- the list says which preview each file gets (asset.preview); FE/BE contract on kinds and header
- the viewer (shared dialog shell): ← / → through the list's such files in its order, Save, open in
  a tab, the file's menu; one stage height for every kind; a no-preview card offers the file
- the name, the card's picture and the menu's Preview open it; an image up to 8 MB is its own
  thumbnail

frontend/src/components/FileCard.tsx

@@ -18,15 +18,16 @@ import {
 import { MoreButton } from "./ui/MoreButton";
 import { Pressable } from "./ui/Pressable";
 
-// A library file in the Files grid: the one media card (ui/MediaCard) — its picture plays it, the
-// title under it, then where it came from and what needs saying about it (kept, leaving soon). The
+// A library file in the Files grid: the one media card (ui/MediaCard) — its picture plays it (a file
+// that is not media: opens it in the viewer), the title under it, then where it came from and what
+// needs saying about it (kept, leaving soon). The
 // right click, the long press and the "…" open the same menu as its table row (DESIGN.md "Files page":
 // the grid is the list's other paint, a toggle remembered per folder).
 
 export function FileCard({
   file,
   index,
-  onPlay,
+  onOpen,
   menuProps,
   moreProps,
   menuOpen,
@@ -35,8 +36,8 @@ export function FileCard({
   file: LibraryFile;
   /** Its place on the page, for the keyboard walk. */
   index: number;
-  /** Null: nothing to play (a plain file, a shared one its owner keeps from me). */
-  onPlay: (() => void) | null;
+  /** Plays it, or opens it in the viewer. Null: a shared file its owner keeps from me. */
+  onOpen: (() => void) | null;
   menuProps: Record<string, unknown>;
   moreProps: Record<string, unknown>;
   menuOpen: boolean;
@@ -47,6 +48,7 @@ export function FileCard({
   const title = jobTitle(file);
   const channel = file.display_uploader || file.asset?.uploader || null;
   const clip = file.job_kind === "edit" || file.job_kind === "clip";
+  const media = file.asset?.media_kind !== "file";
   const thumb = <JobThumb job={file} box="h-full w-full" />;
   return (
     <MediaCard
@@ -55,17 +57,19 @@ export function FileCard({
       className={clsx(arrived && "ring-2 ring-accent/60")}
       picture={
         <div className={mediaPictureBox("wide")}>
-          {onPlay ? (
+          {onOpen ? (
             <Pressable
-              onClick={onPlay}
-              aria-label={t("downloads.actions.play")}
+              onClick={onOpen}
+              aria-label={t(media ? "downloads.actions.play" : "files.actions.view")}
               data-nav-open=""
               className={MEDIA_PICTURE}
             >
               {thumb}
-              <div className={MEDIA_HOVER_LAYER}>
-                <MediaPlayGlyph small />
-              </div>
+              {media && (
+                <div className={MEDIA_HOVER_LAYER}>
+                  <MediaPlayGlyph small />
+                </div>
+              )}
             </Pressable>
           ) : (
             <div className={clsx(MEDIA_PICTURE, "cursor-default")}>{thumb}</div>