fix(content): double-pay is now impossible + purchases auto-file + Paid Files tab

The double-share/double-pay chain (user hit it live, 2026-07-22):
ShareModal's already-shared lookup compared the slash-stripped stored
path against the leading-slash filepath — never matched — so every
re-share minted a NEW catalog entry with a new id, and the buyer's
owned-guard (keyed by id) saw the duplicate as unowned and paid again.
Three independent walls now: (1) ShareModal normalizes both sides so
re-shares reuse the entry; (2) content_server::add_item dedupes by
filename server-side (updates in place, keeps the id so buyers' owned
records stay valid); (3) the buyer REFUSES to pay for content it
already owns — matched by (onion, content_id) OR (onion, filename) —
and serves the cached copy instead, before any ecash is minted.

Purchases also auto-file into Photos/Music/Documents on the node (same
buckets as the Cloud view, collision-safe naming) so bought files show
up where files live on every device, and Cloud gains a Paid Files tab
listing every purchase (name, size, sats paid, date) with in-app view.

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

core/archipelago/src/content_server.rs

@@ -126,12 +126,29 @@ pub fn content_file_path(data_dir: &Path, item: &ContentItem) -> PathBuf {
 }
 
 /// Add a content item to the catalog.
+///
+/// Idempotent per FILE, not just per id: `content.add` mints a fresh UUID on
+/// every call, so id-only dedup let the same file be shared twice as two
+/// separately-priced entries — and a buyer paid twice for one file
+/// (2026-07-22). Same filename → update the existing entry in place and
+/// keep its id, so existing buyers' owned records stay valid.
 pub async fn add_item(data_dir: &Path, item: ContentItem) -> Result<ContentCatalog> {
     let mut catalog = load_catalog(data_dir).await?;
     if catalog.items.iter().any(|i| i.id == item.id) {
         return Err(anyhow::anyhow!("Content item '{}' already exists", item.id));
     }
-    catalog.items.push(item);
+    let norm = |f: &str| f.trim_start_matches('/').to_string();
+    if let Some(existing) = catalog
+        .items
+        .iter_mut()
+        .find(|i| norm(&i.filename) == norm(&item.filename))
+    {
+        let keep_id = existing.id.clone();
+        *existing = item;
+        existing.id = keep_id;
+    } else {
+        catalog.items.push(item);
+    }
     save_catalog(data_dir, &catalog).await?;
     Ok(catalog)
 }