feat(web-ui): add per-file detail panel to Shared Files page - #463

Clicking a shared file in the table opens a detail panel in the lower half of
the page, separated from the table by a draggable splitter, re-fetching live
from GET /shared/{hash} while open. On phones it opens as a full-screen
drill-down sheet, mirroring the Downloads detail panel.

Shared scaffolding:
- Extract the splitter + bottom-panel structure into a reusable SplitDetail
  component consumed by both Downloads and Shared Files.
- Move the detail-panel helpers (magnetLink, copyText, Section, and the stat-row
  builder statRow) into the common components.js module; download-detail.js now
  only exports DownloadDetail. The 'session / total' counter formatter twin()
  moves to format.js so the shared table and detail panel share one copy.
- Rename the split/detail CSS classes that both pages share so they no longer
  carry the download-only dl- prefix (dl-view -> split-view, dl-detail* ->
  detail-*, etc.); the download-only pieces-* graph classes are untouched.

Shared detail panel, organised in three sections with mobile-readable labels
(the per-field tooltips are not reachable on touch):
- Sharing: Size, Uploaded / Requests / Accepted (each '(session / total)'),
  Share ratio, Complete sources. The latter is formatted like the desktop
  column (SharedFilesCtrl.cpp): '< N', 'N', or 'N - N' for the estimate range.
- Activity: Priority, Clients on queue, File type.
- Identity: Hash, Path, Parts.
- Copy ED2K / Copy magnet buttons; Media and Comment sections shown when present.
Shared files have no progress.parts, so there is no pieces graph.

Adds the en/es i18n strings, reusing downloads_detail_* keys where the text
matches.