feat: add calendar docs with inline markdown rendering

Add src/ui/calendar.md with full documentation for both calendar
namespaces (picker props, event grid, ticker, agenda, event data
format, date utilities, CSS classes).

Add a minimal markdown-to-hiccup renderer (ui.markdown) that handles
headings, fenced code blocks, tables, lists, inline code, and bold.
Styled with ui/markdown.css using theme tokens.

Each dev target renders the docs inline on the Calendar page:
- Hiccup: slurps the .md file at render time
- Replicant: embeds via compile-time macro (ui.macros/inline-file)
- Squint: fetches from /calendar.md served by Vite

Also fixes calendar event grid day cells to be square (aspect-ratio: 1
with overflow: hidden instead of min-height).

bb.edn

@@ -20,7 +20,12 @@
              (let [lr (slurp "dev/css-live-reload.js")]
                (spit "dev/replicant/public/css-live-reload.js" lr)
                (spit "dev/squint/public/css-live-reload.js" lr)
-               (println "Copied css-live-reload.js to dev targets")))}
+               (println "Copied css-live-reload.js to dev targets"))
+             (doseq [md (.listFiles (io/file "src/ui")
+                          (reify java.io.FileFilter
+                            (accept [_ f] (.endsWith (.getName f) ".md"))))]
+               (spit (str "dev/squint/public/" (.getName md)) (slurp md)))
+             (println "Copied component docs to dev/squint"))}
 
   watch-theme
   {:doc "Watch src/ui/*.css and tokens.edn, rebuild theme on changes"