terminal: make project mode workspace onboarding clearer

[Senzo13]

Context

Project mode should feel like a local Codex/Claude Code-style flow: choose a workspace, start a project conversation, then use terminal tools only inside that project context.

Relevant files:

• web/static/js/terminal.js
• web/static/js/db.js
• web/static/css/chat.css
• web/routes/terminal.py

Suggested approach

Polish the empty state and workspace picker copy so users understand why a folder is needed and what will happen after choosing it.

Acceptance criteria

• Project mode empty state explains that a workspace is required.
• The flow creates or opens a project conversation after choosing a folder.
• Normal chat conversations do not show terminal-only UI.
• Button labels and warnings use i18n keys.
• node --check web/static/js/terminal.js web/static/js/db.js passes.