feat(dashboard): add writable web terminal action

[nil-err]

Summary

• Keep the existing session terminal entry read-only.
• Add a distinct key-icon action that opens an authenticated writable Web Terminal in a new tab.
• Cover session cards, the details drawer, and Kanban actions; hide the writable action for public read-only access.

Validation

• test/dashboard-sessions-ui.test.ts: 22 passed
• test/dashboard-i18n-c5.test.ts: 430 passed
• test/terminal-write-auth.test.ts: 31 passed
• Full TypeScript build, dashboard bundle, domain audit, and dist audit passed in the local trial worktree.
• Local trial runtime: 6/6 Botmux processes online from the trial worktree; Dashboard root returned HTTP 200 and the unauthenticated sessions API returned HTTP 401.

Scope

This PR contains only the Dashboard terminal-control UI, i18n, style, and focused regression-test changes. No production deployment or remote configuration change is included.