Extract the composer's destination checks into useDestinationIssues #415

Context

resources/js/components/posts/composer/PostComposerDialog.vue is about 3,500 lines. The checks that decide whether a destination can be scheduled are plain script with no template, around lines 1309–1487:

• destinationIssues(account): media warnings, the Instagram image aspect ratio, text and hashtags over the account's limit, missing required settings, and empty or too long thread replies (with their media).
• groupIssues(group) / groupIssueLabel(group): the deduplicated count shown on each network card.
• blockingIssue / hasBlockingIssues: what disables the submit button and fills its tooltip.
• metaRequirement(account), SET_MEDIA_WARNINGS and setMediaWarning(account).

They depend on composition.resolvedDestination(), props.platformConfigs, and the limit helpers remainingCharacters (~L1032), replyLimit (~L1069) and remainingHashtags (~L1289).

This was left in place on purpose to keep #406 focused.

Proposal

• Move these into resources/js/composables/composer/useDestinationIssues.ts, taking the composition and the platform configs as arguments, and move the three limit helpers with them if nothing else in the dialog needs them.
• Keep the DestinationIssue type next to the composable.
• No behaviour change: the composer browser tests (tests/Browser/PostComposerDialogTest.php, ComposerNetworkCardsTest.php, ComposerChannelSettingsTest.php, ComposerMediaTrayTest.php) must pass untouched.