Fix navigator.clipboard crash over plain HTTP (non-secure context)

[constkolesnyak]

Problem

navigator.clipboard is only exposed in secure contexts — HTTPS or http://localhost. When the UI is accessed over plain HTTP via a LAN hostname/IP (e.g. http://192.168.x.x:8900, a .local mDNS name, a Tailscale IP with a self-signed cert, or a proxy without a trusted cert), the entire clipboard object is undefined and any call throws synchronously:

TypeError: Cannot read properties of undefined (reading 'writeText')

Two affected call sites, both silently failed without any UI feedback:

• web/src/components/Chat/CodeBlock.tsx — the per-code-block Copy button.
• web/src/pages/ChatPage.tsx — the Cmd+Shift+C "copy last response" keyboard shortcut.

Same root cause as #149 (which fixed crypto.randomUUID over plain HTTP). Both APIs gate on secure context.

Fix

Add a shared copyToClipboard helper in web/src/utils/clipboard.ts that prefers navigator.clipboard when available and falls back to a hidden off-screen <textarea> + document.execCommand('copy'). execCommand is deprecated but still implemented in every browser we care about.

Both call sites switched to the helper. grep confirms no other navigator.clipboard usages in the frontend.

Style

Mirrors utils/uuid.ts from #149 — same file naming convention, same fallback layering (native → legacy), same "client-side use, no crypto strength needed" reasoning.

Test plan

• Click code-block Copy button under plain HTTP — text lands in clipboard.
• Press Cmd+Shift+C under plain HTTP — last assistant text lands in clipboard.
• Under HTTPS the native navigator.clipboard path is taken — no behavior change.
• npm run build clean.

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