Implement 6-format banner spec, fix CTA colours, add custom sizes

- BannerPreview: 6 real formats (SM2/MD1/MD2/MD3/LG1/LG2) all 312px wide;
  white-pill CTA with theme-colour text (was inverted); navy bg #000063 per
  Barclays spec; link-style CTA for SM2/MD2/MD3; centred icon for LG1;
  photo layouts: photo-left (SM2/MD2), photo-right (MD3), photo-top (LG2)
- Custom size: POST /api/banner-sets/:id/custom-size creates Custom variants
  from each pair's LG copy at user-specified px dimensions; BannerEditor has
  +Custom size form with W×H inputs
- Format limits updated in copy_generation.py (SM2 no body, MD 32/64, LG 64/128)
- tasks.py _persist_banner_set generates all 6 standard formats per copy set
- Migration 0006 adds custom_width/custom_height columns to banner_variants
- VariantsGrid/ChatBrief canProceed uses SMALL/LARGE format sets (not "Medium"/"Large")
- tsc --noEmit clean

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/settings.local.json

@@ -0,0 +1,14 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(git *)",
+      "Bash(./node_modules/.bin/tsc *)",
+      "WebFetch(domain:optical-dev.oliver.solutions)",
+      "Bash(cmux --help)",
+      "Bash(cmux list-windows *)",
+      "Bash(cmux list-workspaces *)",
+      "Bash(cmux list-panes *)",
+      "Bash(cmux list-surfaces *)"
+    ]
+  }
+}