[CodeByArnob]

Summary

Implements the first required part of FoodLens Issue #1: tapping Sign In with Puter or Sign Up with Puter now launches the device's external default browser instead of rendering Puter authentication inside the app.

Changes

• Uses the existing url_launcher dependency with LaunchMode.externalApplication.
• Opens the official Puter login page for sign-in and the official signup page for account creation.
• Removes the embedded WebView authorization modal from the authentication screen.
• Shows an actionable error if Android cannot launch the browser.

Important scope limitation

This PR intentionally handles external browser launch only. It does not claim that the authenticated browser session is already transferred back into FoodLens. The secure callback/session handoff must be implemented only after confirming a Puter-supported native authorization mechanism. Do not remove the remaining bridge/session code until end-to-end authentication, AI usage, and cold-start restoration are verified.

Verification

• Pending CI and physical-device verification.
• Device acceptance: both buttons open the external default browser, and no in-app browser is displayed.

Closes no issue yet because the secure return/session restoration acceptance criteria remain open.