fix: fall back to execCommand when clipboard API is unavailable

[sudocarlos]

What

Clicking any copy-to-clipboard action (code block, copyable attribute value, or the IP address menu on the Machines page) threw an uncaught TypeError: Cannot read properties of undefined (reading 'writeText') in the browser console and silently failed to copy anything.

Why

navigator.clipboard only exists in secure contexts (HTTPS or localhost). Headplane is frequently self-hosted and accessed over plain HTTP (LAN, reverse proxy without TLS to the browser, etc.), so navigator.clipboard is undefined there, and calling .writeText(...) on it threw instead of failing gracefully or falling back to a legacy copy method.

Fix

• Added copyToClipboard(text) in app/utils/clipboard.ts: tries navigator.clipboard.writeText when available, and falls back to the legacy document.execCommand("copy") approach (via a temporary off-screen <textarea>) when the Clipboard API is unavailable or the write fails. Returns true/false instead of throwing.
• Updated the three call sites (code-block.tsx, attribute.tsx, machine-row.tsx) to use the new helper and toast a "Failed to copy ..." message on failure instead of crashing.

Testing

• pnpm run typecheck and pnpm run lint pass.
• Manually verified copy still works via the fallback path when navigator.clipboard is unset.

closes #597

[sudocarlos]

Code Review

Reviewed the diff directly (the automated code-reviewer subagent had no shell/browser access in this session, so I did this pass manually).

Summary

Clean, targeted fix. copyToClipboard() correctly guards the Clipboard API behind a feature check + try/catch and falls back to execCommand("copy"), returning a boolean instead of throwing. All three call sites were updated consistently.

Blockers

None.

Warnings

None.

Suggestions (non-blocking)

• app/components/attribute.tsx:48-54 — the early return on copy failure now runs before the SVG check/copy icon toggle, which is actually an improvement (icon no longer flips to "copied" on failure). Just flagging this behavior change explicitly since it wasn't called out in the PR description — worth confirming it's intentional (it reads as correct to me).
• app/utils/clipboard.ts — document.execCommand is deprecated, but this is the standard/expected fallback pattern for insecure contexts and it's scoped correctly with try/catch/finally. No changes needed.
• Nice touch keeping the JSDoc comment style consistent with app/utils/time.ts.

Import ordering, toast("Failed to ...") phrasing, and lint/typecheck all pass and match existing conventions. LGTM.