fix(mobile): stop cold-launch keyboard flash on explore search

[dylanjeffers]

Summary

• Follow-up to apps#14289. That PR stopped the keyboard from re-appearing on background→foreground, but a brief keyboard flash still happened on app cold launch.
• Root cause: the explore screen's useEffect calls .focus() whenever params?.autoFocus === true, regardless of whether the explore tab is the active tab. If the explore screen mounts off-screen with a stale autoFocus: true on its route, the focus call fires anyway and briefly summons the keyboard.
• Fix: gate the focus call on useIsFocused() so the param is only consumed while the explore tab is actually the active tab. The intentional search-icon-tap flow still works — navigating to explore puts the tab in focus before the effect runs.

Test plan

• Cold-start the app → no keyboard flash on initial load (regardless of which tab was last active before the kill)
• Tap search icon from any tab header → explore opens with keyboard up and input focused
• On explore, dismiss keyboard, switch to another tab, return to explore → keyboard does NOT reappear
• On explore with keyboard up, background the app, return from background → keyboard does NOT reappear (still works after this change)
• Tap search icon → keyboard reappears, dismiss it, tap search icon again → keyboard reappears (param re-trigger still works)

🤖 Generated with Claude Code

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