feat(webview): thinking effort surfaces — header chip, composer toggle, in-chat display (DTE series 4/5)

[easonLiangWorldedtech]

What

The user-visible surfaces for dynamic thinking effort:

• TaskHeader chip — current effective effort + source badge (default / Zoo (auto) / you), visible at task start (init state computed in the webview from settings + model default; extension pushes sync the source badge)
• Composer bottom-bar toggle — between the API-profile selector and the auto-approve (⚡) control; temporary and current-task-only; writes the same task-local state the model tool writes (scope identical)
• In-chat one-line display for model/user changes (switch_mode-style row with partial streaming)
• Extension -> webview state messaging (display copy per tab; authoritative state stays extension-side)

Part of the Dynamic Thinking Effort series (5 PRs). Master design issue: easonLiangWorldedtech#28.

PR protocol (all series PRs): draft PR until CI is fully green -> switch to ready -> CodeRabbit bot review addressed in full -> 100% test coverage on patch lines. Each PR <= 1000 diff lines, based on latest upstream main (stacked on preceding series PRs where noted; rebased to pure main once dependencies merge).

Stacking: PR 4/5 — stacked on PR 3.

Acceptance criteria

• Chip shows the init state at task start and updates with the correct source on every change
• Bottom button changes only the current task; resets on task switch; persisted Settings value never changes
• Menu lists only the model-supported effort levels; adaptive-class models labeled "soft guidance"
• Layout: icon-only trigger with tooltip, flex-shrink-0 + min-w-[28px] (matches AutoApproveDropdown)
• Tests: JSDOM (binding, clamp, disabled, click -> postMessage) + Playwright CT visual snapshot committed for the new user-visible layout (Docker-updated baseline); i18n keys in all required locales
• 100% patch coverage; <= 1000 diff lines