Name the copy keys in the copy failure message #52

Priority: Low
Effort: Small
Requested issue type: Feature

Current behavior

When both copy methods fail, Sitenalyzer shows this status message:

Couldn't copy the ID. Select the displayed ID and copy it manually.

The message says what happened, but "copy it manually" doesn't tell people what to press. The ID is already shown beside the button, so the message can name the keys.

Suggested scope

• Change the failure message in snaCopyToClipboard to name the keys, such as: "Couldn't copy the ID. Select the ID shown and press Ctrl+C (Cmd+C on a Mac)."
• Keep everything else about copying the same. That includes the Clipboard API, the older copy command, the ID shown beside the button, the status area that screen readers announce, and the error timeout from Hide the copy failure message after a timeout #49.
• DataLaVista uses the same wording in its manual copy popup (Improve copying when the clipboard is blocked, and credit a contributor datalavista#52), so the two tools give the same advice.

Acceptance criteria

• After a failed copy, the status message names Ctrl+C and Cmd+C.
• Successful copies still show the same message, such as "List ID copied!".
• Update COPY_FAILURE_MESSAGE in tests/sitenalyzer.spec.js so the clipboard tests expect the new message.
• Update the copy sentence in docs/usage.md, which says the message tells you to "select the displayed ID and copy it manually."

Starting points

• snaCopyToClipboard, where the failure message is shown
• COPY_FAILURE_MESSAGE in the browser tests
• The copy sentence in docs/usage.md

Contributor notes

Read AGENTS.md, SKILLS.md, and the project-preferences skill first. Edit src/, then rebuild dist/ with bash build.sh or .\build.ps1. Run the browser tests from tests/, and update docs/usage.md. Use synthetic examples and preserve SharePoint Content Security Policy support.