Add screenshot post-capture actions

[ryanr14]

Summary

• Add a screenshot post-capture setting for opening the editor, showing the overlay, copying, saving a PNG, or uploading.
• Route screenshot hotkeys, tray actions, and deeplink area captures through the shared post-capture handler.
• Harden one-shot area screenshot deeplinks so they do not persist screenshot mode or leak pending post-capture actions.

Validation

• pnpm exec biome check apps/desktop/src/utils/tauri.ts apps/desktop/src/routes/target-select-overlay.tsx "apps/desktop/src/routes/(window-chrome)/settings/general.tsx" apps/desktop/src/utils/general-settings.ts apps/desktop/src/utils/general-settings.test.ts
• pnpm --dir apps/desktop exec vitest run src/utils/general-settings.test.ts
• pnpm --dir apps/desktop exec tsc -p tsconfig.json --noEmit
• cargo test -p cap-desktop deeplink_actions::tests --lib
• cargo check -p cap-desktop --lib
• git merge-tree "$(git merge-base HEAD origin/main)" HEAD origin/main has no conflict markers locally