Let the sidebar be resized by dragging its edge

The sidebar's right edge is now a splitter, like the one between the
message list and the reading pane: drag it between 240 and 480px, move it
with the arrow keys, double-click to put it back. The width is a device
setting, and stays null until someone drags, so a width set in the
reader's own CSS through --sidebar-w is kept until they choose otherwise.
Hidden on a phone, where the sidebar is a drawer, and while collapsed.

Arrow keys on either splitter moved the pane and never saved it: the
keyboard path called onResize without onEnd. It ends each key press now,
and both views keep the in-progress size in a ref as well as state, so
the end reads the value set in the same tick.

The message-list splitter's accessible name was an untranslated literal.
It goes through translate() now, and it and the sidebar's new name are in
all nine catalogues.

Closes #345

FEATURES.md

@@ -71,9 +71,11 @@ per-account shape.
 ## Layout
 
 Three panes: folder tree, message list, reading pane. The splitter between the
-list and the reading pane is dragged to resize, and the size is remembered per
-device — a width chosen on a 27" monitor is wrong on a laptop, so it is one of
-the few settings that does not follow the account.
+list and the reading pane is dragged to resize, and so is the edge of the
+sidebar, between 240 and 480px; arrow keys move either one, and a double-click
+puts it back. The sizes are remembered per device — a width chosen on a 27"
+monitor is wrong on a laptop, so they are among the few settings that do not
+follow the account.
 
 - **Reading pane** right of the list, below it, or off (messages open full width).
 - **Density** comfortable, cozy or compact, which changes row height as well as padding.