feat(share): inline media preview on single-file share landing

The public share page only rendered media previews for FOLDER shares —
a single-file share got a bare icon + download button, even for images
and videos the browser can play natively.

Backend: resolve the shared file's mime_type + size at read time and
expose them on ShareDto (meta + password-verify endpoints, one shared
enrichment helper). Display-only enrichment: a failed file lookup
leaves the fields None instead of failing the response — the download
endpoint still surfaces the real error.

Frontend: the 'file' view now reuses the folder grid's lazyVideo
(poster-seek + retry) for video and imageRetry for images, with
Range-aware streaming already provided by /api/s/{token}/file/{id}.

Co-Authored-By: Claude Code <noreply@anthropic.com>

frontend/src/lib/api/endpoints/share.ts

@@ -9,6 +9,16 @@ import type { ItemType } from '$lib/api/types';
 export interface ShareMeta {
 	item_type: ItemType;
 	item_name: string;
+	/** The shared item's id — file shares use it to build the preview src. */
+	item_id: string;
+	/**
+	 * File shares only: resolved by the server at read time so the landing
+	 * page can inline a media preview (video player / image) instead of a
+	 * bare download button. Absent for folder shares.
+	 */
+	mime_type?: string;
+	/** File shares only: the shared file's size in bytes. */
+	size?: number;
 }
 
 export interface ShareFolderEntry {

frontend/src/routes/s/[token]/+page.svelte

@@ -287,7 +287,29 @@
 		</form>
 	{:else if view === 'file'}
 		<div class="share__center">
-			<Icon name="file" class="share__big-icon" />
+			{#if meta && mediaKind(meta.mime_type) === 'video'}
+				<!-- Inline player: Range-aware endpoint streams the video, so the
+				     timeline seeks without downloading the whole file first.
+				     lazyVideo defers the load, seeks a few frames in for a poster
+				     and retries once on error (same behaviour as the folder grid). -->
+				<video
+					class="share__media"
+					data-testid="public-share-file-video"
+					use:lazyVideo={shareFileUrl(token, meta.item_id)}
+					controls
+					playsinline
+				></video>
+			{:else if meta && mediaKind(meta.mime_type) === 'image'}
+				<img
+					class="share__media"
+					data-testid="public-share-file-image"
+					src={shareFileUrl(token, meta.item_id)}
+					alt={meta.item_name}
+					use:imageRetry
+				/>
+			{:else}
+				<Icon name="file" class="share__big-icon" />
+			{/if}
 			<h1>{meta?.item_name}</h1>
 			<a
 				class="share__btn"
@@ -486,6 +508,13 @@
 		text-align: center;
 	}
 
+	.share__media {
+		max-width: 100%;
+		max-height: min(70vh, 40rem);
+		border-radius: var(--radius-2xl);
+		border: 1px solid var(--color-border);
+	}
+
 	:global(.share__big-icon) {
 		font-size: 3rem;
 		color: var(--color-text-muted);