fix: rank date sources instead of letting scan order decide

The previous commit had this backwards: the sidecar date was only
consulted when the existing date came from an mtime, so a filename date
silently outranked what Instagram itself reported.

The order is sidecar, then filename, then mtime -- metadata first,
mtime last, since mtime is when the file hit disk and says nothing about
when the post was made. Ties keep the incumbent so two equally
authoritative files cannot flip a post's date by scan order.

Extracted to src/lib/post-dates.ts rather than left inline, because the
rule is easy to state and easy to get wrong -- the tests include an
order-independence case that would have caught the original mistake.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/hooks/useArchiveScanner.ts

@@ -5,6 +5,7 @@ import { ArchiveFile, CacheData, Post, ServerArchive } from '../types';
 import { setCachedArchive, getDirectoryHandle } from '../lib/archive-cache';
 import { parseArchiveFilename, scopedPostId, EXPORT_RE, INSTALOADER_RE } from '../lib/archive-patterns';
 import { isGalleryDlSidecar, sidecarDate, sidecarIsReel } from '../lib/gallery-dl-sidecar';
+import { DateSource, shouldReplaceDate } from '../lib/post-dates';
 
 const hasDirectoryHandle = async (name: string) => Boolean(await getDirectoryHandle(name));
 
@@ -143,8 +144,17 @@ export const useArchiveScanner = (
 
     try {
       const postsMap = new Map<string, Partial<Post>>();
-      // Posts whose date is only a file mtime, so a real date can replace it.
-      const weakDates = new Set<string>();
+      /**
+       * Which source supplied each post's date, so a better one can replace it.
+       * Sidecar beats filename beats mtime — see src/lib/post-dates.ts.
+       */
+      const dateSources = new Map<string, DateSource>();
+      const applyDate = (postId: string, post: Partial<Post>, date: string, source: DateSource) => {
+        const current = post.date ? { date: post.date, source: dateSources.get(postId) ?? 'mtime' } : undefined;
+        if (!shouldReplaceDate(current, { date, source })) return;
+        post.date = date;
+        dateSources.set(postId, source);
+      };
       const mediaFilesMap = new Map<string, ArchiveFile>();
       const discoveredProfilePics: { name: string, url: string }[] = [];
       const allImageFiles: ArchiveFile[] = [];
@@ -300,18 +310,13 @@ export const useArchiveScanner = (
             if (!post) {
               post = { id: postId, date, username: user, caption: '', media: [], isStory, source: kind, highlightTitle: file.source?.title };
               postsMap.set(postId, post);
-              if (parsed.dateFromMtime) weakDates.add(postId);
             }
             else if (isStory) post.isStory = true;
 
-            // A JDownloader highlight filename carries no date, so it is dated
-            // by mtime — but the same item is usually also present under a
-            // gallery-dl name that does carry one. Let the real date win
-            // regardless of which file the scan happened to reach first.
-            if (!parsed.dateFromMtime && date && weakDates.has(postId)) {
-              post.date = date;
-              weakDates.delete(postId);
-            }
+            // Files describing one post are scanned in directory order, not in
+            // order of trustworthiness, so every date goes through the ranking
+            // in post-dates.ts rather than last-write-wins.
+            applyDate(postId, post, date, parsed.dateFromMtime ? 'mtime' : 'filename');
 
             const lowerExt = ext.toLowerCase();
             if (lowerExt === 'txt') {
@@ -326,11 +331,7 @@ export const useArchiveScanner = (
                   const reel = sidecarIsReel(data);
                   if (reel !== undefined) post.isReel = reel;
                   if (data.type === 'story') post.isStory = true;
-                  const day = sidecarDate(data);
-                  if (day && weakDates.has(postId)) {
-                    post.date = day;
-                    weakDates.delete(postId);
-                  }
+                  applyDate(postId, post, sidecarDate(data), 'sidecar');
                 } else if (data) {
                   const node = data.node || data; const iphone = node.iphone_struct || {};
                   const captionText = node.edge_media_to_caption?.edges?.[0]?.node?.text || node.caption?.text || iphone.caption?.text || '';