feat(ui): three button sizes, because the app has three kinds of button

Measured before deciding: across the ~100 bespoke button rules, vertical
padding does not spread — it clusters. ~27 at 0.4–0.45rem, ~28 at 0.25–0.3rem,
~23 at 0.1–0.15rem. Those are three different components that happen to share a
name prefix: a page action, a row action, and an affordance that lives inside a
card.

Collapsing them to the single size the shared sheet had would have visibly
broken every card layout, which is why the one-off migration stopped here for a
decision rather than proceeding on the assumption that a button is a button.

  default        8px 16px   page action — what the house style specifies
  .btn-compact   4px 12px   toolbar, table row, list item controls
  .btn-inline    2px 4px    dismiss ×, confirm tick, add-chip

.btn-small and .btn-sm already sat at the compact step, so they are kept as
aliases for it — no template churn, and the two spellings stop being a third
thing that might drift.

.btn-inline is deliberately below the spacing scale's first step on the
vertical axis: 4px of padding on an 11px label already exceeds the line box
these sit in. Stated in the file so it reads as a measured exception rather
than someone ignoring the scale.

DesignView renders all three as real specimens. A size scale described in prose
is one nobody can check; rendered from the actual classes, it cannot claim
something the app does not do.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/assets/components.css

@@ -105,10 +105,33 @@
   background: var(--color-action-destructive-hover);
 }
 
-/* --- size modifiers ------------------------------------------------------ */
+/* --- size modifiers ------------------------------------------------------
+ *
+ * THREE sizes, because the app genuinely has three. Measured across the ~100
+ * bespoke button rules before this scale existed, vertical padding fell into
+ * clusters rather than a spread: ~27 at 0.4–0.45rem, ~28 at 0.25–0.3rem, ~23
+ * at 0.1–0.15rem. Those are three different components — a page action, a row
+ * action, and an affordance living inside a card — that happen to share a name
+ * prefix. Collapsing them to one size would visibly break the card layouts.
+ *
+ * A button carries its size modifier; the DEFAULT (no modifier) is the page
+ * action, which is the one the house style specifies.
+ */
 
-.btn-small,
+/* Row actions: a toolbar, a table row, a list item's controls. */
+.btn-compact,
+.btn-small,          /* pre-existing spellings, kept so no template churns */
 .btn-sm {
   padding: var(--fs-space-1) var(--fs-space-3);   /* 4px 12px */
   font-size: var(--fs-size-tiny);
 }
+
+/* Inline affordances: a dismiss ×, a confirm tick, an add-chip — things that
+ * sit INSIDE a line of text or a card and must not disturb its rhythm. Below
+ * the spacing scale's first step on the vertical axis by necessity: 4px of
+ * padding on a 11px label already exceeds the line box these live in. */
+.btn-inline {
+  padding: 2px var(--fs-space-1);                 /* 2px 4px */
+  font-size: var(--fs-size-tiny);
+  line-height: 1;
+}