Fix race condition in group join: move state update after database operations

[Copilot]

State was updated before database operations completed. If member addition failed, users would see the map view for a group they never successfully joined.

Changes:

• Moved setCurrentGroup() and setCurrentGroupName() to execute only after all database operations succeed
• Consolidated error handling into single try-catch around all database operations
• Added user-facing error message on join failure

Before:

const code = groupCode.toUpperCase();
setCurrentGroup(code);  // State updated immediately
// ... fetch metadata ...
await database.ref(`groups/${code}/members/${user.uid}`).set({...});  // Could fail

After:

const code = groupCode.toUpperCase();
try {
    // ... fetch metadata ...
    await database.ref(`groups/${code}/members/${user.uid}`).set({...});
    setCurrentGroup(code);  // State updated only on success
    setCurrentGroupName(groupName);
} catch (e) {
    alert('Failed to join group. Please try again.');
}

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