feat(web) (4/5): skill authoring workbench

[zjncs]

Layer (d) of the authoring platform split (iflytek#934 on iflytek/skillhub), stacking on layers (a)–(c) (fork PRs until iflytek#935 lands upstream).

What this layer adds

The web workbench under /dashboard/authoring:

• Draft list + detail (files / runtime binding / validation runs tabs) with a file editor that tracks unsaved state and surfaces optimistic-concurrency conflicts
• Runtime binding form switching fields by agent type (interpreter vs endpoint/model), tool allowlist, MCP server declarations with immediate validation
• Run detail: status card, live event console (SSE with a 2s polling fallback and cursor replay), finding cards with severity, location, suggested-fix diff preview and one-click apply/dismiss
• Submit dialog handing validated drafts to the publish pipeline
• Copy in en/zh/ru; Playwright e2e for the full flow (e2e/authoring-flow.spec.ts)

Verification

• tsc --noEmit clean; 922 web unit tests pass — the only failures in the suite are two pre-existing upstream theme tests that fail identically on plain main (verified in a clean worktree), unrelated to this change
• Unit tests cover the editor's binary-file handling, SSE event merging, patch preview and the validation-stream hook

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