Fix btn-phone rendering when phone is empty (refs #14)

[porcow]

Fixes #14

Title
Fix phone button rendering when no phone is configured

What

• Render the “直接打电话” button only when phone is present
• Prevent empty tel: links in the main page action area

Why

• Avoid showing a non-functional call button when no phone number is set (issue The "btn-phone" button should only be rendered when phone is defined #14)

How

• Wrap the phone anchor in a conditional template so it’s omitted when phone is falsy

Testing

• Not run (manual verification recommended)