feat(web-ui): add per-download detail panel to Downloads page - #461

Clicking a download in the Downloads table opens a detail panel in the lower
half of the page, separated from the table by a draggable splitter; the page
fills the viewport and scrolls its regions internally (app-like) instead of
scrolling the whole page. On phones the detail opens as a full-screen
drill-down sheet over the list (below the top bar) with a pinned close button,
so the tap clearly changes the screen and the info is reachable.

The panel consumes the enriched GET /downloads/{hash} endpoint and updates live
while open (re-fetches on each store tick; ETag-cached).

Top of the panel:
- Completion progress bar.
- Canvas pieces graph mirroring the aMule GUI: green = complete, red = missing,
  blue = available with an intensity gradient by source count (theme-tuned via
  CSS vars). A compact one-line legend shows per-state counts, the total piece
  count, and the availability scale '(fewer -- more)'.
- Copy ED2K link / Copy magnet link buttons on the file-name row. The magnet URI
  matches CamuleAppCommon::CreateMagnetLink; clipboard write falls back to
  execCommand on non-secure origins.

Fields below, ordered by usefulness (status/progress first, technical last) and
each with an explanatory tooltip:
- Transfer: Status, Completed (bytes + %), Speed, Remaining, Sources, Size,
  Transferred.
- Activity: Active time, Last received, Last seen complete, Clients on queue.
- Media / Comment when present.
- Parts: Available parts, Saved by ICH (in packages), Lost to corruption,
  Gained by compression.
- Identity: Hash (uppercase), Met file.

Also: add onRowClick to the shared VirtualTable, a 'copy' icon, widen the
content cap to 1760px, move the Clients nav tab after Shared Files, and the
en/es i18n strings.