fix(web): do not persist untouched empty compose drafts

[Tutitoos]

Problem

Opening a new composer and immediately closing it persists an empty draft. The authenticated inbox then displays a blank conversation row (sender only, no subject or preview), even after refresh. This was reproduced in the deployed UI during the post-merge visual audit of #185.

Scope

• Do not create or checkpoint a draft when a brand-new composer has no recipients, subject, body text, or attachments.
• Preserve recovery and checkpoint behavior for recovered drafts and any composer with meaningful content.
• Keep reply/forward behavior intact.
• Add browser regression coverage for opening and closing an untouched composer.

Acceptance criteria

• Closing an untouched new composer produces no draft API request.
• Leaving an untouched new composer open beyond the local debounce does not create a draft.
• Once meaningful content is entered, autosave/checkpoint behavior remains unchanged.
• Existing web typecheck, unit, build, and browser suites pass.