fix(ui): make copy work in non-secure contexts (plain HTTP / Tailscale)

[MingxiYe]

Summary

Copy buttons in the Web UI fail when the page is served over plain HTTP to a non-localhost host (a LAN IP, or a Tailscale 100.x address). navigator.clipboard is only defined in a secure context (HTTPS, or http://localhost / 127.0.0.1), so every copy path throws:

• hooks/useFunctionPanelData.ts catches it and shows "Failed to copy to clipboard"
• hooks/useCopyFeedback.ts had no .catch and silently no-ops
• the ~15 direct navigator.clipboard.writeText call sites throw

Fix

• Add frontend/src/utils/clipboard.ts copyText(): uses navigator.clipboard.writeText in a secure context, otherwise falls back to the legacy document.execCommand('copy') via an off-screen <textarea>; rejects if neither path can copy.
• Route the copy hooks (useCopyFeedback, useFunctionPanelData) and all direct navigator.clipboard.writeText call sites through it.
• Promotes the correct fallback that already existed locally in components/rule-card/utils.ts into the shared helper (removes the duplicate).

Test plan

• pnpm typecheck: no new errors (40 before this change, 40 after — the repo's pre-existing legacy errors are unchanged).
• pnpm lint: no new findings (only the repo's pre-existing React-pattern warnings remain).
• Secure-context behavior (localhost / HTTPS) is unchanged — it still uses navigator.clipboard.
• The non-secure fallback path (plain HTTP to a non-localhost host, e.g. over Tailscale) runs execCommand('copy'). That branch cannot be exercised from a headless localhost/HTTPS context (where navigator.clipboard is present), so it is verified by review here rather than an automated run.

Fixes #1746

🤖 Generated with Claude Code

[FFengIll]

hwo about to update navigator.clipboard directly ? if we can not, we may create anothe rnew clipboard.