use a custom 404 page

to customize the 404 page you can copy the handlebar template
`src/static/templates/404.hbs` to the TEMPLATES_FOLDER (defaults to
`data/templates/`)

resources/404.svg

@@ -0,0 +1,93 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!-- Created with Inkscape (http://www.inkscape.org/) -->
+
+<svg
+   width="500"
+   height="222"
+   viewBox="0 0 500 222"
+   version="1.1"
+   id="svg5"
+   xml:space="preserve"
+   inkscape:version="1.2.1 (9c6d41e410, 2022-07-14, custom)"
+   sodipodi:docname="404.svg"
+   inkscape:export-filename="404.png"
+   inkscape:export-xdpi="96"
+   inkscape:export-ydpi="96"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:xlink="http://www.w3.org/1999/xlink"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg"><sodipodi:namedview
+     id="namedview7"
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1.0"
+     inkscape:showpageshadow="2"
+     inkscape:pageopacity="0.0"
+     inkscape:pagecheckerboard="0"
+     inkscape:deskcolor="#d1d1d1"
+     inkscape:document-units="px"
+     showgrid="false"
+     inkscape:zoom="1.3791767"
+     inkscape:cx="284.59007"
+     inkscape:cy="214.25826"
+     inkscape:window-width="1916"
+     inkscape:window-height="1038"
+     inkscape:window-x="0"
+     inkscape:window-y="18"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="layer1"
+     showguides="false" /><defs
+     id="defs2"><mask
+       id="holes"><rect
+         x="-60"
+         y="-60"
+         width="120"
+         height="120"
+         fill="#ffffff"
+         id="rect3296" /><circle
+         id="hole"
+         cy="-40"
+         r="3"
+         cx="0" /><use
+         transform="rotate(72)"
+         xlink:href="#hole"
+         id="use3299" /><use
+         transform="rotate(144)"
+         xlink:href="#hole"
+         id="use3301" /><use
+         transform="rotate(-144)"
+         xlink:href="#hole"
+         id="use3303" /><use
+         transform="rotate(-72)"
+         xlink:href="#hole"
+         id="use3305" /></mask></defs><g
+     inkscape:label="Ebene 1"
+     inkscape:groupmode="layer"
+     id="layer1"><rect
+       style="fill:none;fill-opacity:0.5;stroke:none;stroke-width:0.74;stroke-opacity:1"
+       id="rect681"
+       width="666"
+       height="222"
+       x="0"
+       y="0" /><text
+       xml:space="preserve"
+       style="font-size:128px;line-height:1.25;font-family:'Open Sans';-inkscape-font-specification:'Open Sans';text-align:center;text-anchor:middle;fill:#000000;fill-opacity:0.7;stroke-width:1"
+       x="249.9375"
+       y="134.8125"
+       id="text3425"><tspan
+         id="tspan3423"
+         x="249.9375"
+         y="134.8125"
+         style="font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:128px;font-family:'Open Sans';-inkscape-font-specification:'Open Sans';text-align:center;text-anchor:middle;fill:#000000;fill-opacity:0.7;stroke-width:1"
+         sodipodi:role="line">404</tspan></text><text
+       xml:space="preserve"
+       style="font-size:26.6667px;line-height:1.25;font-family:'Open Sans';-inkscape-font-specification:'Open Sans';text-align:center;text-anchor:middle"
+       x="249.04297"
+       y="194.68582"
+       id="text4067"><tspan
+         sodipodi:role="line"
+         id="tspan4065"
+         x="249.04295"
+         y="194.68582"
+         style="font-size:26.6667px;text-align:center;text-anchor:middle;fill:#000000;fill-opacity:0.7">Return to the web vault?</tspan></text></g></svg>