Add prompt stash via long-press on send button

[mcintyre94]

Summary

• Long-pressing the send button shows a "Stash Draft" context menu action (tray.and.arrow.down)
• Saves the current input to a single in-memory stash slot and clears the field
• After any message is sent (whether queued while streaming or dispatched normally), the stash is automatically restored to the input bar
• Matches Claude Code's ctrl+s stash behaviour — no explicit restore UI needed

Test plan

• Type a long prompt, long-press the send button, tap "Stash Draft" — field should clear
• Type a quick question, send it — stash should immediately restore to the input bar
• Stashing again while something is already stashed overwrites the previous stash
• Long-pressing when the field is empty shows no context menu
• Works on iPad/Mac (context menu is right-click on Mac)
• Unit tests pass: stashDraft_movesInputTextToStash, stashDraft_doesNothingWhenEmpty, stashDraft_overwritesPreviousStash, stashDraft_leavesInputReadyForNextMessage

🤖 Generated with Claude Code

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.