Default the Ctrl/Cmd+F search shortcut to in-room search on desktop

[hayaksi1]

Problem

The ctrlFForSearch setting (Ctrl/Cmd+F opens in-room message search instead of the
browser's find-on-page) shipped off by default for everyone and with no description,
so the shortcut most users expect to search the conversation did nothing useful out of the
box. On the desktop app there is no browser "find on page" for it to clash with, and Seshat
makes encrypted-room search work, so defaulting it off there is a missed default. On web the
opposite is true — overriding the browser's native Ctrl+F is surprising. The setting therefore
wants different defaults on the two platforms (#24359), and a description so users understand
what it does (#22888).

Fix

• Default ctrlFForSearch to on for the desktop app (!!IS_ELECTRON) and keep it off on
  web, so the desktop app searches the room by default while web preserves the browser's
  native find bar.
• Add a description to the setting explaining what the shortcut does and that encrypted-room
  search needs the desktop app with message search enabled.

No new UI surface — this is a default + copy change on the existing setting, confined to
Settings.tsx and the i18n string. The settings-tab snapshot is regenerated to include the
new description.

Tests

• New KeyBindingsDefaults-test.ts: the Ctrl/Cmd+F room-search keybinding is registered when
  ctrlFForSearch is enabled and omitted when it is disabled.
• PreferencesUserSettingsTab snapshot updated for the added description.

Fixes #22888
Relates to #24359

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)