fix: confirm task copy only after clipboard success

[ooiuuii]

Why

The Tasks Copy button immediately reports 'Copied task line' while its parent discards the clipboard Promise. Both desktop and mobile task surfaces can therefore report success for a pending write, a rejection, or an unavailable clipboard API.

Approach

Await a boolean result from the existing copyTextToClipboard helper (including its existing fallback) before displaying success. Clear prior feedback on a new click and use a per-row request counter to discard obsolete completion feedback and unmounted-row results. Preserve exact raw markdown selection and existing error logging. One existing owner file changes; no dependency or server changes.

Validation

New Chromium checks on 2026-10-09 against main f8f1343:

• RED 02:22:07–02:22:10 UTC: six false-success assertions, pending/rejected/missing API on desktop and mobile
• GREEN 02:22:34–02:22:45: both actual surface components cover pending, rejection, missing API, exact raw line, success, repeated actions, overlapping writes, latest success, 1.5-second feedback expiry and settlement after unmount/remount. 26 observations, no page errors
• Failure/delayed-success cases inject navigator.clipboard and force the fallback to return false. Separate user-click controls call Chromium's real writeText, which fulfilled on both surfaces; OS clipboard readback/paste was NOT tested
• pnpm install --frozen-lockfile, pnpm check, pnpm build, web-source TypeScript and git diff --check passed
• One scoped review found no blockers; no subsequent source changes

Browser proof mounts real TaskGraphDrawer/MobileTasksSection and shared TaskGraphContent/TaskItem with fixture markdown over loopback HTTP. No backend mutation is involved. Mobile means the actual mobile component in a 390x844 Chromium viewport, not physical-device/WebKit proof. Initial external harness parsing/serialization faults were corrected before the valid untouched-main RED; no product changes preceded RED.

Runnable diagnostic retained outside the production diff under the current UI testing exception. Not run: full pnpm test, whole-project tsc, full-app navigation/production CSS, OS clipboard readback, real mobile/Windows/macOS, release/package/native matrix. Existing style/build warnings retained; no full-suite or remote-CI pass claimed.