fix(composer): show the autosave copy, and drop a posted draft

[jouwdan]

Closes #275

Two composer draft fixes.

1. Autosave strings rendered as raw keys

ComposerRecovery is one island rendered by both composers. It reads six keys from its copy prop — composer.autosave.saving/saved/failed and composer.recovery.available/restore/discard. replyFormCopy() built all six; newThreadFormCopy() built none, so the moment a new thread autosaved, the page printed composer.autosave.saved verbatim under the message box. pnpm i18n:check couldn't catch it: the catalog carries the messages, the page just never asked for them.

• apps/community/src/view/content-copy.ts — the six keys are one shared list now, spread into both records so they can't diverge.
• apps/community/src/view/content-copy.test.ts — scans the composer components for every fromCopy(copy, …) they read and asserts the record a page hands them carries each key and resolves it to prose. Fails without the fix.

2. A posted draft was offered on the next thread

Posting cleared the server draft, but two paths put it back, so starting another thread in the same forum offered the one just posted:

• A Server Action's redirect is a client-side navigation with JS on, so the browser's recovery copy in localStorage was never removed on a successful post — pagehide doesn't fire.
• The blur that pressing Post fires raced an autosave in behind the action's own drafts.remove, recreating the server draft after it had gone.

apps/community/src/components/content/composer-recovery.tsx now removes the browser copy when the composer unmounts mid-submit, and a capture-phase pointerdown on any submit button holds off the blur autosave until the next keystroke — so the post's draft removal is the last word. Typing after a failed submit clears the hold, so an errored attempt keeps its draft.

e2e/composer-drafts.spec.ts covers it: autosave a thread, post it, and the next new-thread page is empty with no recovery offer and no localStorage copy left.

docs/guides/community/formatting.md gains a Drafts section describing the feature and this behaviour.

Validation

pnpm lint, pnpm typecheck:app, pnpm i18n:check, pnpm docs:links:check, pnpm docs:index:check, pnpm comments:check, the content-copy unit test, and the new e2e all pass.