Give the cards an edge again — one grey, not ten hues

The border was never the problem; a border that carried COLOUR was. It said
exactly what the fill already said, at 1.56-2.09 against that fill where the
fill managed 1.03-1.05 against the board — the loudest element on every card
was redundant with the quietest. A line that varies by colour is content and
competes with the fill. A line that never varies is structure and does not.

So the edge comes back, and it comes back as a constant in NoteCard rather
than a column in the palette. Uniformity is the feature, and putting it where
the palette cannot reach it is how that stays true.

  light  #b8b8b8    1.57-1.98 against all twenty card fills
  dark   #404040    1.58-1.73

Matched, not eyeballed: both land at ~1.6-1.7 against the card they edge, so
the edge reads with the same authority in either theme. Dark is `neutral-700`
— what the `default` card's border always was, one entry's value promoted to
the rule for all of them. Light sits between `neutral-300` and `neutral-400`
because neither lands in range: 300 fades to 1.18 on a gray-tagged card, 400
jumps to 2.52 and reads as a wireframe.

Rejected on measurement: a translucent black/white edge, which is the tidier
way to write it and self-adjusts per card. A border composites over the
card's own fill, so `border-white/20` comes out #56396d on a purple card and
#a3c9c1 on a teal one. Hue-coded edges are the thing being removed.

The shadow steps back to what it was for — depth, not the boundary. Web
returns to `shadow-sm`; Android's 2dp drops to 1dp, matching it.

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

frontend/src/components/NoteCard.vue

@@ -222,7 +222,7 @@ onBeforeUnmount(() => document.removeEventListener("mousedown", onDocMousedown))
 <template>
   <div
     ref="root"
-    class="group relative mb-4 break-inside-avoid rounded-xl p-3 shadow transition hover:shadow-md"
+    class="group relative mb-4 break-inside-avoid rounded-xl border border-[#b8b8b8] p-3 shadow-sm transition hover:shadow-md dark:border-[#404040]"
     :class="[
       noteCardClasses(note),
       dragging ? 'opacity-40' : '',
@@ -233,14 +233,30 @@ onBeforeUnmount(() => document.removeEventListener("mousedown", onDocMousedown))
     ]"
     :data-note-id="note.id"
   >
-    <!-- No `border`. The hue border that used to frame every card outshouted the fill
-         it framed and turned the board into a grid of identical outlines — see the
-         measurements above NOTE_CARD_CLASSES. `shadow` rather than `shadow-sm` because
-         it is now the ONLY thing separating a light-mode card from the board: an
-         untagged card is `bg-red-50` on `bg-neutral-50`, a 1.05 contrast that reads as
-         a card purely by sitting above one. Dark mode gets no shadow worth the name
-         against a near-black board, which is why the subdued fills moved onto the card
-         surface instead. -->
+    <!-- THE EDGE LIVES HERE, NOT IN THE PALETTE, and that is the whole point of it.
+         Every card gets the same grey hairline whatever colour it is; the fill is the
+         only thing that varies. The version of this that was a `{hue}-900` border
+         failed because the line was both the loudest element on the card (1.56-2.09
+         against its own fill, where the fill managed 1.03-1.05 against the board) AND
+         carried the same information the fill did, so a board of them read as a grid
+         of outlines. A neutral line carries no information at all, which is exactly
+         what lets it be structure.
+
+         The two values are MATCHED, not picked by eye: each measures ~1.6-1.7 against
+         the card it edges (light 1.57-1.98, dark 1.58-1.73), so the edge has the same
+         authority in either theme. #404040 is `neutral-700`, which is what the default
+         card's border always was — promoted from one entry in the palette to the rule
+         for all of them. #b8b8b8 sits between `neutral-300` and `neutral-400`, neither
+         of which lands in range: 300 fades out at 1.18 on a gray-tagged card, 400
+         jumps to 2.52 and reads as a wireframe.
+
+         NOT a translucent black/white edge, which is the tidier-looking way to do this
+         and was measured and rejected: a border composites over the card's own fill,
+         so `border-white/20` comes out #56396d on a purple card and #a3c9c1 on a teal
+         one. Hue-coded edges are the thing being removed.
+
+         `shadow-sm`, back down from `shadow`: the border is the boundary again, so the
+         shadow is only depth. -->
     <img
       v-if="firstImage"
       :src="firstImage.url"