Rebuild the web UI on the Cinder design

Replaces the card-and-chip visual language with the Cinder direction from
the design doc: editorial serif, containerless sheets divided by ash
hairlines, one 760px measure, no radii and no shadows.

The organising rule is that heat is typographic. Only a series with an
unread chapter is allowed to be crimson — title, cover foot rule, "Ch N
out", play icon — so the one thing worth acting on is the one thing that
draws the eye. Favourites get brass instead of borrowing the accent, which
is what made the old three-hue palette read as noise. Both states hang off
two classes on the article (is-new, is-dim), so a new sub-element inherits
the state rather than re-deriving it.

The action strip is flex-basis 100% inside the row, which is what lets the
same markup be a full-width six-cell strip under the row on a phone (46px
targets) and a group of 40px squares beside it on a desktop, with no
duplicate template branches. Icons move to a sprite in icons.html: cards
are swapped in by htmx and reference the page's symbols, so a row no
longer carries a screenful of inline SVG paths.

[hidden] gets a display:none override because every disclosure panel is
now a flex container and display beats the attribute. filter.js grows an
.open class alongside hidden so the strip can show which cell the open
panel belongs to.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/templates/icons.html

@@ -0,0 +1,15 @@
+{{define "icons"}}
+<!-- One sprite per page. Card fragments swapped in by htmx reference these
+     symbols by id, so the markup for a row stays a handful of <use> tags. -->
+<svg width="0" height="0" style="position:absolute" aria-hidden="true">
+  <symbol id="i-play" viewBox="0 0 24 24"><path d="M8 5l11 7-11 7z" fill="currentColor"/></symbol>
+  <symbol id="i-star" viewBox="0 0 24 24"><path d="M12 3.6l2.6 5.6 6 .8-4.4 4.2 1.1 6-5.3-2.9-5.3 2.9 1.1-6-4.4-4.2 6-.8z" fill="none" stroke="currentColor" stroke-width="1.7" stroke-linejoin="round"/></symbol>
+  <symbol id="i-star-on" viewBox="0 0 24 24"><path d="M12 3.6l2.6 5.6 6 .8-4.4 4.2 1.1 6-5.3-2.9-5.3 2.9 1.1-6-4.4-4.2 6-.8z" fill="currentColor" stroke="currentColor" stroke-width="1.7" stroke-linejoin="round"/></symbol>
+  <symbol id="i-pencil" viewBox="0 0 24 24"><path d="M4 20h4L19 9l-4-4L4 16z" fill="none" stroke="currentColor" stroke-width="1.7" stroke-linejoin="round"/></symbol>
+  <symbol id="i-box" viewBox="0 0 24 24"><path d="M3 7h18v4H3zM5 11v9h14v-9M10 15h4" fill="none" stroke="currentColor" stroke-width="1.7" stroke-linejoin="round"/></symbol>
+  <symbol id="i-check" viewBox="0 0 24 24"><path d="M4 12.5l5.2 5.5L20 6.5" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/></symbol>
+  <symbol id="i-trash" viewBox="0 0 24 24"><path d="M4 7h16M9.5 7V4h5v3M6.5 7l1 13h9l1-13M10.5 11v6M13.5 11v6" fill="none" stroke="currentColor" stroke-width="1.7" stroke-linecap="round"/></symbol>
+  <symbol id="i-undo" viewBox="0 0 24 24"><path d="M4 9h9.5a5 5 0 010 10H8M4 9l4.2-4.2M4 9l4.2 4.2" fill="none" stroke="currentColor" stroke-width="1.7" stroke-linecap="round" stroke-linejoin="round"/></symbol>
+  <symbol id="i-search" viewBox="0 0 24 24"><circle cx="10.5" cy="10.5" r="6.5" fill="none" stroke="currentColor" stroke-width="1.8"/><path d="M15.3 15.3L20 20" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round"/></symbol>
+</svg>
+{{end}}