feat: Excalidraw + Banner plugins + ITS CSS snippets

Plugins installed:
  obsidian-excalidraw-plugin
    manifest.json + styles.css tracked in git
    main.js (8MB) excluded from git — downloaded by setup-vault.sh on first run
  obsidian-banners
    manifest.json + main.js + styles.css all tracked (148KB, acceptable)

CSS Snippets (ITS Theme by SlRvb):
  ITS-Dataview-Cards.css  — transforms TABLE queries into visual card grids
    Usage: use backtick-dataview-cards instead of backtick-dataview above query
  ITS-Image-Adjustments.css — fine-grained image sizing in notes
    Usage: caption images with |100 or wrap in image-adjustments callout

Config updates:
  community-plugins.json — added obsidian-excalidraw-plugin, obsidian-banners
  appearance.json        — added ITS-Dataview-Cards + ITS-Image-Adjustments snippets
                           (vault-colors preserved)
  .gitignore             — added excalidraw main.js exclusion rule

setup-vault.sh — step 5: auto-downloads Excalidraw main.js if missing
  curl from GitHub releases/latest, skips if already present

Canvas updates (Obsidian saved state):
  welcome.canvas — user added Cosmic Brain GIF at top
  main.canvas    — user added brand assets to Wiki Pages zone
  Wiki Map.canvas — Obsidian format normalised

.gitignore

@@ -6,6 +6,9 @@
 !.obsidian/plugins/calendar/data.json
 !.obsidian/plugins/thino/data.json
 
+# Excalidraw main.js is 8MB — downloaded by setup-vault.sh, not tracked in git
+.obsidian/plugins/obsidian-excalidraw-plugin/main.js
+
 # Large binary assets — add your own images to _attachments/images/
 # Uncomment the line below to exclude images from git (recommended for large vaults)
 # _attachments/images/