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

web/src/locales/nl.ts

@@ -1181,6 +1181,8 @@ export const catalog: Catalog = {
     "Maximize": "Maximaliseren",
     "Full screen": "Volledig scherm",
     "Resize panes": "Vensterdelen verslepen",
+    "Resize message list": "Grootte van de berichtenlijst wijzigen",
+    "Resize sidebar": "Grootte van de zijbalk wijzigen",
     "QR code": "QR-code",
     "Draft discarded": "Concept weggegooid",
     "Draft saved": "Concept opgeslagen",