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).

src/ui/calendar.cljc

@@ -1,4 +1,5 @@
 (ns ui.calendar
+  "Month-grid date picker. See src/ui/calendar.md for full documentation."
   (:require [clojure.string :as str]
             [ui.button :as button]
             [ui.icon :as icon]))