Replace static path composition with named routes in Vue Router navigation

## What

Throughout the frontend we have ~37 places where Vue Router navigation uses hardcoded path strings instead of named routes. This includes `router.push('/')`, `router.push({ path: '/admin/users' })`, template literal composition like `` `/team/${slug}/billing` ``, relative paths like `{ path: 'general' }`, and string-based redirects in route definitions.

All of these should use named routes with params instead, e.g. `router.push({ name: 'home' })` or `{ name: 'team-billing', params: { team_slug: slug } }`.

## Why

Static path composition is fragile. If a route path ever changes, every hardcoded reference breaks silently. Named routes decouple navigation from URL structure, which is the whole point of having route names. We're already standardizing route names in #8084, so this is the natural follow-up.

## Prerequisites

- #8084 (Standardize Vue Router route names to kebab-case) should be completed first, since we'll be referencing the new kebab-case names.

## How

Broken down into sub-issues ordered by blast radius (smallest first). Each sub-issue is a self-contained PR.

## Sub-issues

- [ ] #8095 — Replace string-based redirects in route definitions with named routes
- [ ] #8096 — Replace root path and absolute path navigation with named routes
- [ ] #8097 — Replace relative path navigation in settings pages with named routes
- [ ] #8098 — Replace template literal / string concat path composition with named routes