fix(browser-search): restore frecency function removed in #434

[monotykamary]

Problem

Commit 8992618 (PR #434 — "combine browser and root search improvements") deleted the frecency() helper function but kept two call sites in the new getLegacyCompletion() function, causing a ReferenceError: frecency is not defined at runtime when the legacy completion path is hit (i.e. when alphaChromiumRootSearchEnabled is off).

Fix

Restore the accidentally deleted frecency(entry: BrowserSearchEntry) function:

function frecency(entry: BrowserSearchEntry): number {
  const ageDays = Math.max(0, (Date.now() - entry.lastUsedAt) / (24 * 60 * 60 * 1000));
  const recencyFactor = 1 / (1 + Math.log10(1 + ageDays));
  return entry.useCount * recencyFactor;
}

This is the exact implementation that was removed — no logic changes.

Testing

• TypeScript compiles without errors
• The legacy browser search autocomplete path no longer throws ReferenceError