Adds a combobox-style filter to the posts index toolbar so users can
narrow All / Scheduled / Posted / Drafts views by one or more labels.
- `PostController::index` accepts `?labels[]=<id>` and applies
`whereHas('labels', whereIn(...))` (OR semantics across selected labels).
Workspace labels are exposed to the page (sorted by name) and the
selected set comes back under `filters.labels`.
- New `LabelFilter.vue` component reuses the existing Popover + Command
pattern (matching `FontPicker` in the Brand settings page). Trigger
renders the selected `LabelBadge`s inline (mirroring how each post row
already displays its labels): 1-3 shown directly, 4+ shown as the
first three plus a "+N" overflow indicator. Clear button has a
tooltip and `cursor-pointer`, and stops `click`/`pointerdown`/
`mousedown` so it doesn't reopen the Popover.
- Existing search debounce is shared with the new label watcher via a
single `buildFilterUrl` helper. URL is updated with `preserveState +
replace` so the back stack stays clean.
- i18n in en / pt-BR / es: `filter_by_label`, `label_search_placeholder`,
`no_labels`, `clear_label_filter`.
Tests: 4 new index tests covering the labels prop exposure, single-label
filter, multi-label OR filter, and blank-id sanitization. Full suite:
1509 passed, 2 skipped, 0 failed.
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