Add support for sending folders via drag & drop

[1270011]

Closes #290

This implements folder sending as specified in #290:
- Dropped directories (onto the page or onto a peer) are read recursively using the
File and Directory Entries API using the approach referenced in the issue, with the
two known pitfalls handled: all entries are gathered synchronously before the drop
handler yields (the DataTransferItemList is emptied afterwards), and
readEntries() is called in a loop since it returns at most 100 entries per call.
- A new dialog then lets the user choose whether to send each folder as a ZIP
archive (directory structure preserved) or to send the contained files
individually - the optional chooser suggested in the issue.
- Zipping reuses the already bundled zip.js in store mode (level 0) via a dedicated
ZipWriter, so it cannot interfere with the shared writer used by the receive-side
"download as zip". Progress is shown on the peer icon (peer drop) or via a
persistent notification (share mode).
- The transfer protocol, the server and the receiving side are unchanged - fully
compatible with older clients and the Android app.

 Robustness details:
 - Unreadable entries (file deleted after drag start, broken symlinks) are skipped with
   a console warning instead of aborting the whole drop.
 - The drop guards are re-evaluated after the asynchronous traversal, so a dialog that
   opened in the meantime is not covered/focus-stolen.
 - Empty folders produce no zero-byte archives; loose files dropped alongside folders
   are sent unchanged.
 - Fixes a small pre-existing bug on the way: the "dropped on peer" guard in
   `PeersUI._onDrop` used `$$('x-peer').contains(e.target)`, which only checks the
   *first* peer element   a drop onto any other peer was handled twice (peer send +
   share mode activation). Now uses `e.target.closest('x-peer')`.

 Not included, to keep the main flow simple:
 - Folder selection via the file picker (`webkitdirectory` forces a folder-only picker,
   so it would need a second input).
 - Streaming the archive during transfer - the ZIP is currently materialized before
   sending, like the receive-side zip; streaming would require protocol changes.

 Translations: new strings are in `en.json`; a separate commit adds the German
 translations - feel free to drop it if translations should go through Weblate only.

 Tested manually on a deployed instance with Chromium and Firefox clients (folders with
 subfolders and >100 entries, folders mixed with loose files, empty folders, Escape to
 cancel, language switching while the dialog is open) plus automated tests for the
 traversal logic (readEntries batching, nested paths, `webkitGetAsEntry` fallback,
 unreadable entries).