@grackle-ai/server: Unescaped Error String in renderPairingPage() HTML Template

Summary

### Impact

The `renderPairingPage()` function embeds the `error` parameter directly into HTML without escaping: ```typescript const errorHtml = error ? `<p style="color:#e74c3c">${error}</p>` : ""; ```

All current call sites pass hardcoded strings, so this is **not exploitable today**. However, the function is architecturally fragile — if a future code change passes user-controlled or dynamic content into the error parameter, it would create an XSS vulnerability.

The `renderAuthorizePage()` function in the same file correctly uses `escapeHtml()` for dynamic content, making this an inconsistency.

**Affected code:** - `packages/server/src/index.ts:64-89` — `renderPairingPage()` with unescaped error interpolation - Compare: `packages/server/src/index.ts:130` — `renderAuthorizePage()` correctly uses `escapeHtml()`

### Patches

v0.70.1

**Fix:** Apply `escapeHtml()` to the error parameter: ```typescript const errorHtml = error ? `<p style="color:#e74c3c">${escapeHtml(error)}</p>` : ""; ```

### Workarounds

No workaround needed — all current callers pass hardcoded strings.

### Resources

- CWE-79: Improper Neutralization of Input During Web Page Generation - File: `packages/server/src/index.ts`