Fix router issues

[MetRonnie]

• Set page title per-route rather than per-view

  • Multiple views may be included in a page, so the previous approach was flawed
  • This removes a blocker from fix(deps): update dependency vuetify to v3.6.8 #1785 (for some reason that Vuetify update has uncovered this flaw)

• Improve router error handling & tidy

• Address this warning that has been around for ages:

  [Vue Router warn]: <router-view> can no longer be used directly inside <transition>
  

• Removes @unhead/vue dependency as it is a bit overkill for just setting the page title, which can be done simply using document.title = title

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Tests are included
• Changelog not needed as not user-facing

[oliver-sanders]

(Not too fussed about loosing the fade transition)