Fix localStorage access crash under browser Tracking Prevention

[Copilot]

Browsers with Tracking Prevention (Edge) or ITP (Safari) throw when localStorage is accessed in cross-origin contexts (e.g. after redirect from the external auth service), crashing the app before the user can log in.

Changes

• src/lib/login.js — Added exported safeLocalStorageGet / safeLocalStorageSet helpers that catch storage-access exceptions and warn to console; replaced bare localStorage calls in loginUser() with these wrappers. Exported so other modules can reuse them.
• src/Signin.svelte — Wrapped localStorage.setItem("google-token", ...) in handleGoogleCredential with try/catch (inline <svelte:head> script cannot use ES module imports, so the guard is applied directly).

// Before — throws in Edge/Safari with Tracking Prevention active
localStorage.setItem("user", JSON.stringify(newOrExistingUser));

// After — degrades gracefully; user is sent through auth flow on next load
safeLocalStorageSet("user", JSON.stringify(newOrExistingUser));

When storage is blocked, the app logs a warning and falls through to the normal auth redirect rather than crashing.

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