tiles: several documents open at once

`L` or Shift-Enter on a row opens it in a tile of its own; Enter and `l`
still open it in the focused tile. Right of the list the tiles share the
space as hyprland's dwindle does: two side by side, a third splits the
newer one, and so on. Each keeps its own scroll position and its own
editor, and the keys act on the focused one. A document already open in a
tile is focused there rather than opened twice, so two drafts of it can
never overwrite each other.

Alt-h/j/k/l moves the focus to the tile that way, out of the editor too,
and Alt-h past the leftmost tile focuses the list as before. With Shift
the tile itself moves, swapping places with its neighbour. `gt` / `gT` and
`:bn` / `:bp` cycle the tiles, `:ls` lists them, `:b <n or name>` focuses
one, `:close` (Alt-q) closes one, asking first when its draft could not be
written, and `:only` closes the rest. `:b` was the board's alias; `:board`
is spelled out now. `:next` / `:prev` still walk the list. `q` on a
document closes its tile when there are several. `f` fullscreens the
focused tile.

Each space remembers its tiles, as it remembered its one document. A
card's notes open in a tile beside whatever was being read, and `q` there
takes the tile away again on the way back to the board. Under 768px only
the focused tile is shown. The status line says which tile has focus.

web/src/lib/Help.svelte

@@ -7,8 +7,12 @@
     ['Shift 1 … 9', 'send the marked nodes, or the selected one, to that space'],
     ['Tab · Shift-Tab', 'next / previous space'],
     ['w', 'overview of every space — hjkl, then Enter'],
-    ['f', 'fullscreen the focused pane; again or Esc to leave'],
-    ['Alt-h / Alt-l', 'focus the list / the document, from the editor too'],
+    ['f', 'fullscreen the focused pane or tile; again or Esc to leave'],
+    ['Alt-h/j/k/l', 'focus the tile that way, from the editor too — left of the leftmost is the list'],
+    ['Alt-Shift-h/j/k/l', 'move the focused tile that way'],
+    ['L · Shift-Enter', 'open the row in a new tile beside the others'],
+    ['gt / gT', 'next / previous tile — in the editor too'],
+    ['Alt-q', 'close the focused tile — :close; :only closes the others'],
     ['j / k · ↓ ↑', 'move down / up the list — scroll the document when it has focus'],
     ['gg / G', 'first / last row, or top / bottom of the document — a first g, d, ] or [ waits a second in the status line'],
     ['Ctrl-D / Ctrl-U', 'half a page down / up the open document'],
@@ -31,7 +35,7 @@
     ['Ctrl-P', 'find anything, in every space — Enter goes to the result lit'],
     ['b', "open this space's board"],
     ['c', "capture a card into this space's inbox, without leaving here"],
-    ['Esc · q', 'close the document'],
+    ['Esc · q', 'close the document, and its tile when there are several'],
     ['/', 'full-text search, inside the current space — Tab or Ctrl-N/P walk the results, Enter opens one'],
     ['n / N', 'next / previous search result, opened beside the results — :noh clears the search'],
     ['s', 'toggle sharing for the document — asks before taking it back'],
@@ -74,6 +78,7 @@
     tree: 'spaces & folders',
     board: 'boards',
     documents: 'documents',
+    tiles: 'tiles',
     editing: 'writing',
     sharing: 'sharing & export',
     vault: 'vault',