feat(webserver): add tooltips to buttons and clickable images in default template - #216

…ult template

Add a descriptive title attribute to every button and clickable image in
the default webserver templates so their purpose is shown on hover, and
improve the alt text on action images for accessibility. All tooltips are
in English to stay consistent with the rest of the (hardcoded) UI.

- dload: pause, resume, raise/lower priority, cancel and filter buttons
- shared: reload, raise/lower priority and apply-priority buttons
- search: Search and Download submit buttons
- servers: network Disconnect button and the per-server connect/remove
  image links (generated in PHP, now with title + alt)
- kad: connect from known peers, disconnect, connect IP and update URL
- login: log-in submit button
- footer: 'Download link' submit button
- prefs: Apply submit button

Only HTML attributes were added; no structural, JS, CSS or backend
changes.