fix(web/desktop): restore agent mode selector and Linux old-layout eligibility

[rasraz]

Restores the composer agent selector (build/plan) when only built-in agents are present, and makes existing Linux/macOS desktop installs eligible to opt back into the previous dashboard.

• app: isAgentsVisible shows the selector whenever more than one selectable agent exists (matches the legacy composer).
• desktop: old-layout eligibility now also checks the previous Tauri data directory.

Closes #39665
Refs #38111

[github-actions]

Hey! Your PR title fix(web/desktop): restore agent mode selector and Linux old-layout eligibility doesn't follow conventional commit format.

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