Fix #1558: close blank draft when opening Drafts

[Keegil]

Enhancement — Closes blank compose drafts automatically when navigating to the Drafts list.

Problem

When a user clicked Compose and then opened the Drafts folder without typing anything, the blank compose pane remained open in the drafts list instead of being dismissed. This made the drafts view cluttered and inconsistent. Issue #1558.

Fix

• Detect and close blank unsaved drafts before navigating to the Drafts list
• Treat default signature-only content as blank so untouched drafts are dismissed
• Leave replies, forwards, and prefilled drafts unaffected — only truly empty drafts are closed

Testing

• Unit tests added for blank-draft detection logic in draftdesk.service.spec.ts
• Cypress regression test added for the Compose → Drafts navigation flow
• npx tsc -p src/tsconfig.spec.json --noEmit passes

Closes #1558