feat: redesign content editor previews

docs/TASKS/content/008-multi-channel-preview-editor.md

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+# Task: Redesign content editor for shared multi-channel previews
+
+## Feature
+
+`docs/FEATURES/channels.md`
+
+## Goal
+
+Make content creation feel like planning a real social post instead of filling out legacy form fields.
+
+The editor should use one shared content body for every selected target channel, show selectable target channels as a vertical tab-style list, and preview the same content in platform-shaped cards for networks such as YouTube and X.
+
+## Scope
+
+- Keep content item editing in `frontend/src/features/content/views/ContentItemDetailView.vue`.
+- Replace per-channel caption editing with a shared caption and shared hashtags.
+- Let configured workspace channels be selected or unselected as targets.
+- Show selected targets in a vertical rail and render preview cards that look closer to social platform previews.
+- Convert shared hashtags into chip-style entry with suggestions from already used hashtags.
+- Show a workspace hashtag feed so authors can reuse existing tags.
+- Preserve existing save payloads and backend contracts.
+
+## Validation
+
+```bash
+cd frontend
+npm run build
+```
+
+## Acceptance Criteria
+
+- [x] Content authors select target channels with checkbox-like controls.
+- [x] The same shared content appears on every selected channel preview.
+- [x] The editor includes platform-specific preview treatment for YouTube and X/Twitter.
+- [x] Saving still creates or revises a content item with the selected targets.
+- [x] Hashtags commit to removable chips as they are entered.
+- [x] Already used hashtags are available as suggestions.
+- [x] The editor shows a workspace hashtag feed with usage counts.