feat(01-02): add Button, Card, Badge templ components + CSS

- button.templ: Button(ButtonProps) renders <button type=...> with class
  from ui.ButtonClass(); Attrs spread for hx-* pass-through
- button.css: .ui-button base + .ui-button-solid-default-md variant
  with non-nested :hover and :focus-visible (Codex concern #7)
- card.templ: Card(attrs) accepts children via templ child syntax
- card.css: slate-50 panel, slate-200 border
- badge.templ: Badge(BadgeProps) renders <span class=...>
- badge.css: info / success / danger variants (warning deferred)

backend/internal/web/ui/badge.css

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+/* badge.css — Phase 1 ships info / success / danger.
+ * `warning` is declared in variants.go for forward compatibility but has no
+ * CSS rule in Phase 1 — it lands when a page first needs it.
+ */
+
+.ui-badge {
+  display: inline-block;
+  padding: 0.125rem 0.5rem;
+  font-size: 0.75rem;
+  font-weight: 600;
+  border-radius: 0.25rem;
+  line-height: 1.25;
+}
+
+.ui-badge-info {
+  background-color: #dbeafe;
+  color: #1d4ed8;
+}
+
+.ui-badge-success {
+  background-color: #dcfce7;
+  color: #16a34a;
+}
+
+.ui-badge-danger {
+  background-color: #fee2e2;
+  color: #dc2626;
+}

backend/internal/web/ui/badge.templ

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+package ui
+
+// BadgeProps is the input to the Badge templ component.
+type BadgeProps struct {
+	Label   string
+	Variant BadgeVariant
+}
+
+templ Badge(props BadgeProps) {
+	{{ class := BadgeClass(props.Variant) }}
+	<span class={ class }>{ props.Label }</span>
+}

backend/internal/web/ui/button.css

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+/* button.css — Phase 1 ships only solid / default / md.
+ * Codex concern #7: no CSS nesting (`&:hover`). All pseudo-class rules are
+ * declared as top-level selectors so the file is portable across every
+ * Tailwind v4 standalone processing mode.
+ */
+
+.ui-button {
+  display: inline-flex;
+  align-items: center;
+  font-family: inherit;
+  font-size: 1rem;
+  line-height: 1.25;
+  border: 1px solid transparent;
+  cursor: pointer;
+  user-select: none;
+  text-decoration: none;
+}
+
+.ui-button:focus-visible {
+  outline: 2px solid #1d4ed8;
+  outline-offset: 2px;
+}
+
+.ui-button.htmx-request {
+  opacity: 0.6;
+  pointer-events: none;
+}
+
+.ui-button-solid-default-md {
+  display: inline-flex;
+  align-items: center;
+  border-radius: 0.375rem;
+  background-color: #2563eb;
+  padding: 0.5rem 1rem;
+  font-size: 1rem;
+  font-weight: 600;
+  color: #ffffff;
+}
+
+.ui-button-solid-default-md:hover {
+  background-color: #1d4ed8;
+}
+
+.ui-button-solid-default-md:focus-visible {
+  outline: 2px solid #1d4ed8;
+  outline-offset: 2px;
+}

backend/internal/web/ui/button.templ

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+package ui
+
+// ButtonProps is the input to the Button templ component.
+//
+// Type defaults to "button" when empty. Attrs is a pass-through for arbitrary
+// attributes (notably hx-* HTMX attributes).
+type ButtonProps struct {
+	Label   string
+	Variant ButtonVariant
+	Tone    ButtonTone
+	Size    Size
+	Type    string
+	Attrs   templ.Attributes
+}
+
+templ Button(props ButtonProps) {
+	{{ btnType := props.Type }}
+	if btnType == "" {
+		{{ btnType = "button" }}
+	}
+	{{ class := ButtonClass(props.Variant, props.Tone, props.Size) }}
+	<button type={ btnType } class={ class } { props.Attrs... }>{ props.Label }</button>
+}

backend/internal/web/ui/card.css

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+/* card.css — slate-50 panel with slate-200 border. */
+
+.ui-card {
+  border-radius: 0.5rem;
+  border: 1px solid #e2e8f0;
+  background-color: #f8fafc;
+  padding: 1.5rem;
+}

backend/internal/web/ui/card.templ

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+package ui
+
+// Card is a slate-50 panel with a slate-200 border. Children are rendered
+// inside via templ's child-content syntax: `@ui.Card(nil) { <p>...</p> }`.
+//
+// `attrs` is a pass-through for arbitrary attributes (id, data-*, hx-*).
+templ Card(attrs templ.Attributes) {
+	<section class="ui-card" { attrs... }>
+		{ children... }
+	</section>
+}