[no bump] Move technical docs to docs/, add icon and PNG export tool

README now carries the icon, TL;DR, description and features; the
technical sections live in docs/. tools/icon_png.py renders
images/icon.svg to a square PNG (512x512 by default, --resolution N).
plugin.json points at the plugin's own icon.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/how-it-works.md

@@ -0,0 +1,16 @@
+# How Message Logger works
+
+## Server sync and caching
+
+The server only points at copies (logger/activity group message ids); the client reads them
+through its own session, so the account must be a member of those groups.
+
+The server is asked about a chat only when needed: when it wasn't checked within the last hour
+(from the oldest loaded message on; the answer replaces the chat's cached marks), when older
+history is loaded past the checked range (that part only), or after an edit/delete event in the
+chat. Answers hold at most 100 marks (edited/deleted) and 100 missing ids, newest first. Marks
+are cached per chat in the plugin's SQLite (`state.db`, no size cap), so a chat opened again —
+even offline — shows them at once, and deleted messages are restored from the cache. An expired
+chat keeps its cache until the next successful check, which replaces it and starts over. Cached
+answers older than *Cache lifetime* (1/7/30/90/365 days or never; 30 by default) are removed,
+checked once a day (from local midnight) and when the setting changes.