Redesign the downloads list as a file-explorer-style table with a two-bar toolbar

[PureJoyMind]

Summary

Rework the Downloads page from the current card list into a file-manager-style table, and
split the toolbar into two bars. Big change; landing as one PR with many small commits.

List / table

• Columned table. Column options: Name (always visible, always leftmost, not
  hideable/reorderable), Type (file extension), Size, Created, Speed,
  Progress %, Progress size. Default visible: Name, Size, Progress %, Speed, Created.
• Click a header to sort by that column; click again toggles asc/desc. Default: Created,
  newest first.
• Columns can be resized (header grips) and reordered (drag the header, or a
  "Move left / Move right" entry in the header's right-click menu). Name stays pinned.
• Chosen columns + sort order persist across restarts; widths do not.
• Right-click the column header → menu to show/hide columns (file-explorer style) + move.

Rows

• Per-row selection checkbox + click / Ctrl+click / Shift+click selection, header
  select-all. Selected rows are highlighted.
• All 5 action icons stay inline (pause, resume, cancel, open-folder, remove), in a
  fixed-width area inside the Name cell, before the filename. Filename ellipsizes when tight;
  tooltip shows the full name, tooltip on the path shows the full path.
• The per-connection row expansion is removed. The single aggregate progress bar (shown
  for active rows only) instead fills unevenly — each connection fills its own region of the
  one bar. Unknown-size downloads keep the indeterminate bar.
• The per-download speed-limit editor moves into the Speed column cell, revealed on
  hover/focus (must not change row height).
• Right-click a row → open file / open containing folder / copy download link / remove.
  Acts on the whole selection when the clicked row is selected.

Toolbar (two bars)

• Bar 1: search (left) · advanced-add + button (opens the existing Add Download
  dialog) · quick-add link box with a clipboard-paste icon · a start button · settings gear
  (right). A trash icon appears next to the gear whenever ≥1 row is selected; the
  Delete key does the same.
• Bar 2: the sortable column header.
• Quick-add uses defaults only (default folder, default connection count, no speed limit),
  clears the box on success, and falls back to opening the advanced dialog (pre-filled) on an
  invalid URL or a conflict.

Bulk delete

One confirmation dialog listing the N filenames, with a single "Also delete files from disk"
checkbox — same semantics as removing a single download.

Status filter tabs

Removed — sorting + search replace them.

Design sketch and a file-explorer reference screenshot to be added under docs/.