fix: theme, sidebar and desk icon for the backup screens on Frappe v16

[tulayha]

Fixes how the backup screens look and navigate on Frappe v16.

Dark mode

In dark mode the Backup Control page's cards and snapshot table rendered white, their text barely visible, and the selected day in the schedule editor was dark on dark.

• Cards and table: Frappe v16 themes only its own .frappe-card; Bootstrap's .card compiles to background-color: #fff. The page's cards now use the desk's --card-bg and --border-color.
• Day buttons: use Frappe's btn-primary / btn-default instead of custom colors (--gray-300, and --fg-color text on --primary-color, which is dark on dark).

Sidebar, breadcrumbs and desk icon

• Two sidebars: the workspace was named "Restic Backups" while the module is "Restic Backup". v16 generates a sidebar for every module that has none named after it, so the doctypes got a generated "Restic Backup" sidebar and the page got "Restic Backups". The workspace is now named after the module, and the app ships its own "Restic Backup" sidebar (Backup Control, Backup Runs, Backup Settings) and desk icon. A patch removes the entries named "Restic Backups".
• Stale sidebar: v16 chooses a page's sidebar from the module of the last doctype opened, so opening Backup Control from another app (for example through Ctrl+K) kept that app's sidebar until reload. The page now chooses its sidebar again when shown.
• Breadcrumbs: the runs list and settings no longer replace their breadcrumb with "Backup Control", and the page no longer adds "Setup". Their "Backup Control" buttons stay.
• Desk icon: an original solid and subtle icon.

The Docker suite passes on v16 and v15. Checked in the browser on a copy of production: every route to the backup screens now shows the one "Restic Backup" sidebar.

[tulayha]

🎉 This PR is included in version 0.1.4 🎉

The release is available on GitHub release

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