[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 +1,33 @@
+# How Unlocked Restrictions works
+
+## How copying works
+
+Telegram refuses to forward protected content, and also to re-send its media by file id, so a
+copy is sent the same way as a message you wrote:
+
+- text keeps its formatting; photos, videos, files, voice messages and GIFs are downloaded
+  (or taken from the local cache) and uploaded again; stickers, locations and contacts are sent
+  directly;
+- albums are sent as media groups again (at most 10 items per group);
+- bot-only parts are dropped: reply and inline keyboards, games, invoices; content that can't
+  be reproduced (polls, dice, stories, giveaways, paid media) is skipped, and only its text is
+  sent if there is any;
+- the selection is sent once every item is ready, so one large download delays the batch;
+- items that can't be copied are reported in a bulletin.
+
+Copied videos have no preview thumbnail. The lock covers groups and channels, not private chats
+where the other person enabled content protection.
+
+## Runtime notes
+
+Things the on-device runtime does differently from the SDK docs, all handled in `src/`:
+
+- Java lists are not iterable from Python; use `hooking.java_list()`.
+- `param.setResult()` in a before-hook does not skip the original method, so the forward hook
+  empties the message list instead.
+- `threading.local` state is not shared between a before-hook and its after-hook.
+- Java `String` values arrive as Python `str`, other `CharSequence`s as Java objects.
+- Fields named like `java.lang.Object` methods (`SendMessageParams.notify`) must be set with
+  `hook_utils.set_private_field`.
+- Exceptions escaping a Python thread vanish silently, so the copy worker catches everything.
+- Shared TL objects (`TLRPC.Chat` and friends) are never modified for display purposes.