feat(downloads): one-click Share + downloadable-by-default links (E7 S3)

D2-A: the library row's Share button becomes a single action — quickShareLink()
ensures a public link (minting on demand), copies it to the clipboard, and toasts
with an "Open in new tab" action. A copy failure still toasts (warning) with that
action so the link is never a dead end. Advanced controls (password/expiry/revoke/
user-ACL) stay in the existing ShareDialog, reached via a new "Manage sharing" row
button.

D1 (forward-only): CreateLinkIn.allow_download defaults to true, so a newly minted
link is downloadable by default. The default lives in the API, not a migration —
the download_links column default stays false, so no existing link flips. The
ShareDialog create-default is flipped to match.

Tests: TestCreateLinkDefault (default true; explicit false honored); 4 vitest cases
for quickShareLink (mint-vs-reuse, copy ok/fail, create failure). HU/EN parity.

backend/tests/test_links.py

@@ -93,6 +93,24 @@ class TestPublicMetaAudioFlag:
         assert meta["audio"] is False
 
 
+class TestCreateLinkDefault:
+    """D1 (E7 S3): a newly minted share link is downloadable by default. The one-click Share relies on
+    this — it POSTs no `allow_download`, so the API default decides. Forward-only lives here (the API
+    default), NOT in a migration: the DownloadLink column default stays False, so no existing back-
+    catalogue link silently flips to downloadable."""
+
+    def test_omitted_allow_download_defaults_downloadable(self):
+        from app.routes.downloads import CreateLinkIn
+
+        assert CreateLinkIn().allow_download is True
+
+    def test_explicit_false_is_respected(self):
+        # The owner can still uncheck it to mint a stream-only link.
+        from app.routes.downloads import CreateLinkIn
+
+        assert CreateLinkIn(allow_download=False).allow_download is False
+
+
 class TestIsExpired:
     def test_none_expiry_never_expires(self):
         assert is_expired(SimpleNamespace(expires_at=None)) is False