feat(editor): per-platform media assignment UI

[Cryptoom]

Summary

• Adds a media toggle grid to each platform card in the post editor (Edit.vue), letting the user narrow which of the post's media items a specific platform publishes. Default is "all on" (empty pivot = applies to every media item, matching A1's data model and today's behaviour).
• New platformMediaIds state threads through PostEditorTabs to ScheduleTab to ChannelConfigurator to new MediaAssignmentGrid.vue, mirroring the existing platformContentTypes/platformMeta pattern.
• Backend contract, confirmed compatible with A4's MCP work (PR feat(mcp): per-platform media parameters for attach and update tools #13): UpdatePostRequest validates platforms.*.media_ids against the media on this same request (IDOR guard), UpdatePost action syncs the PostPlatform::media() pivot (Arr::has distinguishes an omitted key, left untouched, from an explicit empty-array clear). PostController::edit() appends media_ids per PostPlatform instance only (not globally via $appends), so Index/Calendar listings that don't eager-load media aren't hit with an unwanted lazy load under strict model mode.
• i18n: posts.edit.media_assignment.label/included/excluded added to all 16 locales.
• Deliberate scope boundary: this PR is web-UI only. The public REST API and MCP tools getting media_ids/post_platform_ids support is A4's PR feat(mcp): per-platform media parameters for attach and update tools #13, not duplicated here.

Review fixes (second commit)

Two real findings from the first review round, both fixed:

1. Toggle-off-everything bug: deselecting the last remaining media item on a platform flipped the array back to [], which means "unscoped, applies to all" everywhere it's read, silently re-including every item instead of leaving none selected. Fixed by keeping the last remaining item on: a platform always publishes at least one media item once scoped.
2. Missing card wrapper: MediaAssignmentGrid.vue rendered as a bare div, so two selected platforms produced two identical, unbordered blocks with no visible way to tell which belonged to which platform. Now uses the same card pattern as sibling settings panels (DiscordSettings.vue et al) with a platform icon header, and the checkmark badge reuses the existing Badge variant="success" token instead of duplicating its classes.

Screenshot review

Verified with Playwright against a locally seeded post (2 platforms, 3 media items):

• Desktop (1920x1080): both platform cards are now visually distinct (own border, own platform icon), scoped platform shows the excluded item dimmed, unscoped platform shows all included.
• Deselecting item 1, then item 2, then item 3 on a 3-item post: the third click is blocked, item 2 stays on, confirming the toggle-off fix.
• Toggling persists across a full page reload (round tripped through the real validation plus pivot sync path, not mocked).
• Mobile (iPhone 15 Pro, 393x852): same grid renders correctly under the Channels mobile tab, touch targets measured at 56x56px, well above the 44px minimum.
• No em-dashes.

Test plan

• vendor/bin/pest tests/Feature/PostControllerTest.php tests/Feature/UpdatePostRequestTest.php tests/Feature/LocalizationParityTest.php (121 passed)
• Full --filter Post suite (983 tests) green after rebase onto A2 (fix(posts): per-platform media validation instead of shared list #11) and A3 (refactor(publishers): use PostPlatform::scopedMediaItems() instead of shared post media #10)
• vendor/bin/pint --format agent passed
• vue-tsc --noEmit zero errors in touched files
• New tests: platform media_ids scoping, IDOR rejection of foreign media ids, empty-array clears scoping, omitted key leaves scoping untouched, edit() page serializes media_ids per platform
• Code review plus design review, round 2: both fixes verified live with Playwright before merge

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