Fix BrowserScreen back button: step through page history before closing

Reasoned through the code rather than reproducing live (per explicit
instruction). An earlier change made back always close the whole browser
screen in one press, regardless of how many pages deep the session was --
on the theory that stepping back one page at a time through a long
search/login session was tedious. That trade was wrong: it meant a single
accidental back press (system gesture or the in-app arrow) could silently
destroy an entire multi-page browsing session with no way to recover it,
and it doesn't match how every other browser or in-app browser view on
the platform behaves.

Now: go back through the WebView's own page history first
(canGoBack()/goBack()), and only close the screen once there's nowhere
left to go back to. Both the system back gesture and the in-app top-bar
arrow route through the same handleBack(), so they behave identically.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ7ikw2ukGJFTHE3bJS7zL

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