style(visual-fidelity): components — batches B,E,G,H

- B: token-equivalent rgba → color-mix in SignupFlowOverlay, OnboardingWidget
- E: drop text-white Tailwind utility from ImageUpload remove-button (now color: var(--parch-text) inline)
- G: typography off-scale snaps (9→10, 14→13, 15→16, 19→18 px)
- H: padding off-scale snaps in BoardPostCard/Form, CirclePicker, FilterBar, LoginModal

app/components/BoardPostCard.vue

@@ -158,7 +158,7 @@ const slackLinks = computed(() => {
 <style scoped>
 .board-post {
   border: 1px dashed var(--border);
-  padding: 18px 22px;
+  padding: 20px 24px;
   background: var(--surface);
   break-inside: avoid;
   -webkit-column-break-inside: avoid;
@@ -219,7 +219,7 @@ const slackLinks = computed(() => {
 
 .post-title {
   font-family: "Brygada 1918", serif;
-  font-size: 19px;
+  font-size: 18px;
   font-weight: 500;
   color: var(--text-bright);
   margin: 0 0 12px;