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/app/routes/downloads.py

@@ -823,7 +823,12 @@ def list_links(
 
 class CreateLinkIn(BaseModel):
     password: str = ""
-    allow_download: bool = False
+    # D1 (E7 S3): a newly shared download is downloadable by default — the common intent is "let the
+    # recipient have the file", not stream-only. Forward-only: this only sets the default for links
+    # minted from now on; existing links keep whatever `allow_download` they were created with (the
+    # column default stays False, so no back-catalogue link silently flips). The owner can still
+    # uncheck it in the Share dialog, and toggle any link stream-only afterwards.
+    allow_download: bool = True
     expires_days: int | None = None