Resolve all README FIXMEs and enforce truncation safety

- Update Architecture tree to match actual src/ structure
- Fix settings button to have border and hover state (Clickable Affordance)
- Cap truncateMiddle to remove at most 10 chars (anti-spoofing guard)
- Raise caller floor from 10 to 32 chars for address display
- Fill in default RPC URL (ethereum-rpc.publicnode.com)
- Fix dependencies table intro (four runtime libs, not two)
- Clean up TODO section: remove all completed items

src/popup/index.html

@@ -23,7 +23,7 @@
                 </h1>
                 <button
                     id="btn-settings"
-                    class="bg-transparent border-none text-fg cursor-pointer text-2xl p-0 leading-none"
+                    class="border border-border text-fg cursor-pointer text-xl px-1 leading-none hover:bg-fg hover:text-bg"
                     title="Settings"
                 >
                     &#9881;