Port upstream #600: configurable Enter behavior

[bruno320x]

Clean compatibility port of spacedriveapp#600 against integration/upstream-prs.

The original PR depended on a newer @spacedrive/ai ChatComposer contract. The current staging branch already uses @spacedrive/ai ^0.2.5, so the dependency is no longer pinned at the old ^0.2.3 level cited upstream.

Adds persisted Enter-to-send vs Enter-for-newline preferences, Settings > Composition, matching behavior in PortalComposer and CortexChatPanel, and modifier-submit hints. Final acceptance is gated by frontend typecheck/build to prove the current package exposes enterToSubmit.

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
🔒 Security Review	✅ Completed 2026-09-15T18:05:37.613435Z	9199c50	PR opened

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