refactor: improve donation tabs code organization

- Extract logic into dedicated functions for better readability
- Create handleDonationIntervalChange() as main click handler
- Separate concerns: updateLinkInterval, updateAmountDisplay, switchActiveTab
- Remove unnecessary comments in favor of clear function names
- Update footer to link script.js instead of donation-tabs.js

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.claude/settings.local.json

@@ -0,0 +1,11 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(git add:*)",
+      "Bash(git commit:*)",
+      "Bash(cat:*)"
+    ],
+    "deny": [],
+    "ask": []
+  }
+}