feat(issues): add an always-visible completion action

[Dante-dan]

What does this PR do?

Adds an always-visible green completion action to issue details. Clicking it saves the Done status before dismissing the card; a failed save keeps the issue open. Regular pages return to the issue list, previews close, and standalone desktop issue windows close.

The existing action was only visible when a host supplied onDone, and dismissed immediately after starting the mutation. This change keeps the existing issue mutation and status behavior rules, adds pending feedback, and supplies dismissal callbacks at the missing host boundaries. Already-done issues close without another write.

Related Issue

Closes #9009

Type of Change

• New feature (non-breaking change that adds functionality)

Changes Made

• Show the shared completion button on every issue detail, with a green semantic token, spinner, and disabled pending state.
• Dismiss only on successful completion; preserve the existing error handling.
• Wire preview and standalone desktop dismissal through the shared route.
• Add regression coverage for save-before-close, failed saves, already-done issues, and preview dismissal.

How to Test

1. Open an active issue as a regular detail page, preview, or standalone desktop window and click Mark as done.
2. Confirm the status saves before the page/card/window closes. Reopening the issue shows Done.
3. When a save fails, confirm the issue stays open and the existing error toast appears. For an already-done issue, confirm dismissal does not send another update.

Verified revision: 9a8fefcd203079f8a21d4876d3a0f35ec628114a.

• Root pnpm typecheck: 10/10 tasks successful (Turbo cache).
• Root pnpm lint: 7/7 tasks successful, with existing warnings.
• Existing issue-detail, issue-peek, and issue-detail-route suites with one worker: 129 tests passed.
• Root pnpm test: failed on a 5-second text-contrast timeout and Vitest fork startup timeouts. The original text-contrast suite passed independently with one worker (23 tests).

This is a draft while visual QA evidence is completed. The completion behavior has component coverage; full backend/E2E verification remains outstanding.

Checklist

• I have included a thinking path that traces from project context to this change
• I have run tests locally and they pass
• I have added or updated tests where applicable
• If this change affects the UI, I have included before/after screenshots
• I have updated relevant documentation to reflect my changes
• I have considered and documented any risks above
• I will address all reviewer comments before requesting merge

AI Disclosure

AI tool used: OpenAI Codex (gpt-6.1-sol)

Prompt / approach: Under the user's direction to follow up eligible issues, I used the feature request in #9009 to implement an always-visible completion button, traced the shared view and its web/desktop hosts, reviewed mutation success/error handling, and ran the repository checks and regression suites. This describes the AI-assisted approach, not a verbatim historical user prompt.

Screenshots (optional)

Before/after screenshots are pending; the checklist remains unchecked.

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