fix: validate destination address on send view

- Validate Ethereum addresses (0x + 40 hex chars) and ENS names
- EIP-55 checksum validation for mixed-case addresses
- Block sending to zero address (0x0000...0000)
- Warn when sending to own address (allow but show warning)
- Inline error messages with reserved space (no layout shift)
- Disable Review button while address is invalid

Closes #67

src/popup/index.html

@@ -496,6 +496,11 @@
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
                         placeholder="Address (0x...) or ENS name"
                     />
+                    <div
+                        id="send-to-error"
+                        class="text-xs"
+                        style="min-height: 1.25rem; color: #cc0000"
+                    ></div>
                 </div>
                 <div class="mb-2">
                     <div class="flex justify-between mb-1">