Add a copyable deep link for the selected Primary and Compare runs

[engkimo]

Problem

Operators currently build or copy a run deep link from the browser address bar. It is easy to share the wrong Primary run after switching between a synthetic demo, a real call, and a comparison.

Goal

Provide an explicit copy action that preserves the selected Primary and optional Compare run IDs.

Suggested scope

• Add a Copy link control near the run selector or summary.
• Build the URL from the current run_id and optional compare_run_id state.
• Preserve the current path and omit empty query parameters.
• Show short accessible success/failure feedback without blocking the page.
• Do not copy credentials, audio-session tokens, API base internals, or unrelated form state.

Likely files:

• web/src/App.tsx
• web/src/styles.css

Acceptance criteria

• Primary-only links reopen the same run.
• Primary+Compare links reopen both selections.
• Empty Compare is omitted.
• The control is keyboard accessible and has a useful accessible name.
• Clipboard failure produces a visible safe message.
• npm --prefix web run build passes.

Out of scope

• URL shorteners.
• Authentication or public sharing of protected recordings.
• Persisting UI state beyond the two run IDs.

This is a bounded good first issue with an operator-visible result.