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).

dev/replicant/src/dev/replicant.cljs

@@ -20,7 +20,9 @@
             [ui.icon :as icon]
             [ui.separator :as separator]
             [ui.calendar :as calendar]
-            [ui.calendar-events :as cal-events]))
+            [ui.calendar-events :as cal-events]
+            [ui.markdown :as markdown])
+  (:require-macros [ui.macros :refer [inline-file]]))
 
 ;; ── State ───────────────────────────────────────────────────────────
 
@@ -390,9 +392,13 @@
    ["Dev & Technical"
     [:code :terminal :database :globe :shield :zap :book-open :map-pin]]])
 
+(def calendar-docs-md (inline-file "../../src/ui/calendar.md"))
+
 (defn calendar-page []
   [:div
    (page-header "Calendar" "Date picker, event grid, ticker strip, and agenda list.")
+   (into [:div {:class ["md-docs"]}]
+         (markdown/markdown->hiccup calendar-docs-md))
    (calendar-demo)])
 
 (defn icons-page []