[ajslater]

Summary

Adopts Vuetify 4.3's labs VCheckboxGroup in six places, one commit per component so any one can be reverted on its own.

• Labs registration: vuetify({ autoImport: { labs: true } }). A group hands its own (possibly unset) color to its boxes, so VCheckboxGroup defaults to primary like VCheckbox. Groups line up with lone checkboxes app-wide: global.scss drops the labs CSS's 16px label and 6px box indents and spaces inline boxes 16px apart.
• Derived formats: the write formats, titled by comicbox, are added to tagging-choices.json instead of being hand-written in two components.
• Fix: require a default write format. ComicboxTaggingDefaultsSerializer.validate_default_formats rejects an empty list, a non-list and unknown ids. comicbox refuses a write with no formats, and the admin tab autosaves, so unchecking the last format used to break every online-tagging write.
• Edit Tags → Formats: checkboxes instead of a chip select. The last checked format is disabled and the group says why; this replaces enforceMinFormat and its undeclared lastFormats. On md and up the hint sits beside the boxes, so the toolbar stays one 64px row; on phones it stacks and the toolbar wraps.
• Admin → Tagging → Metadata Formats: same at-least-one rule; the docs links stay in the hint (messages, since hint is one string). A row already saved empty shows an error instead of autosaving a default.
• Online Tag → Sources: a source without an API key is disabled with a hint pointing to Admin → Tagging; an empty selection explains why Search is disabled.
• Table column picker: one group per category. Clicks still go through toggleColumn, so a new column lands in canonical position.
• Browser drawer → Show Collection Levels: one click still sends one setSettings call with only the flag it flipped.
• Admin → Defaults → Show Collections: the locked top collection's reason is the group's hint.

vuetify-split-btn makes the same one-line vite.config.js change, so whichever merges second applies cleanly.

Test plan

• New specs: edit-panel-formats, browser-settings-collection; new cases in tagging-tab, launcher-dialog, browser-table-column-picker-standalone, defaults-tab. Each first mount asserts the VCheckboxGroup component resolves. Backend: empty, non-list and unknown formats are 400s.
• make lint, make ty, make test-frontend, make build-frontend, make test pass.
• Checked all six in the dev stack at desktop and phone width: alignment, disabled-box hints, Space toggles a focused box, each group's accessible name.

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