Add quick actions to editor chat, make DraftChatPanel view-agnostic

[oandregal]

Summary

• Make DraftChatPanel view-agnostic. Replaced the isProjectView boolean with three explicit props — placeholder, quickActions, and welcomeText — so the component no longer knows which view it lives in. Callers decide what to show.
• Add editor quick actions. The editor chat now shows "Make it shorter", "Improve clarity", and "Suggest a stronger opening" as quick-action buttons in the empty state, with a welcome message.
• Fix source files showing project chat. Switched the condition from docKind to openResource so source files (not just drafts/done) get the editor chat variant.
• Persist file card on quick-action messages. Quick actions explicitly attach the open file as a visible chat attachment so the reference survives navigation away from the file.
• Remove "Summarize my sources" from the project-view quick actions.

Test plan

• Open a draft → editor chat shows "How can I help with this?" + 3 action buttons
• Open a source file → same editor chat (not project chat)
• Open a done file → same editor chat
• Click a quick action → file card appears in the user bubble
• Navigate away from the file → file card persists in chat history
• Project view (no file open) → shows "What would you like to write?" + project actions
• Verify "Summarize my sources" is no longer in project actions

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