fix(web): extract single tryGetHostname helper (DRY, unified fallback)

Fixes #1368

Previously,  was duplicated in 4 places with 3 subtly different fallback behaviors:
1. inline-citation.tsx: returned  on error
2. markdown-text.tsx: returned  on error
3. assistant-message.tsx: returned  on error
4. citation.tsx: returned  on error

Created canonical  in  that:
- Returns
- Strips  prefix from hostname
- Returns  on invalid URL (safest contract)

Updated all 4 call sites:
- inline-citation.tsx:  (preserves original fallback)
- markdown-text.tsx:  (preserves original fallback)
- assistant-message.tsx:  (drop-in, both return )
- citation.tsx:  (drop-in, both return )

Co-authored-by: guangyang1206 <guangyang1206@users.noreply.github.com>

surfsense_web/lib/url.ts

@@ -0,0 +1,14 @@
+/**
+ * Extract a normalized hostname from a URL. Strips a leading `www.`.
+ * Returns `undefined` if the input is not a parseable URL.
+ *
+ * This is the canonical replacement for the four previously-duplicated
+ * `extractDomain` helpers that had subtly different error fallbacks.
+ */
+export function tryGetHostname(url: string): string | undefined {
+    try {
+        return new URL(url).hostname.replace(/^www\./, "");
+    } catch {
+        return undefined;
+    }
+}