Fix user timezone preference handling #91

Description

The current timezone preference system uses a mix of short codes and IANA timezone identifiers, with manual conversion between them. This should be modernized to use standard IANA timezone identifiers throughout.

Current state

• Timezone preferences stored in u_users.timezone column
• Uses a mix of short codes (e.g., "US/Pa") and full IANA identifiers
• Manual conversion between short codes and full identifiers
• No validation of timezone identifiers
• No handling of historical timezone changes

Impact

• Inconsistent timezone storage format
• Manual maintenance of timezone code mappings
• Potential for invalid timezone identifiers
• No handling of historical timezone changes
• DST transitions may be handled incorrectly

Required changes

1. Database schema changes:

   -- Add validation constraint to ensure valid IANA timezone identifiers
   ALTER TABLE u_users 
   ADD CONSTRAINT valid_timezone 
   CHECK (timezone ~ '^[A-Za-z0-9_+-/]+$');

2. Migration steps:

   -- Create temporary column for new timezone format
   ALTER TABLE u_users ADD COLUMN timezone_new VARCHAR(50);
   
   -- Migrate existing timezone values
   UPDATE u_users 
   SET timezone_new = CASE 
     WHEN timezone = 'US/Pa' THEN 'America/Los_Angeles'
     WHEN timezone = 'US/Ne' THEN 'America/New_York'
     -- Add other mappings
     ELSE timezone 
   END;
   
   -- Verify migration
   SELECT timezone, timezone_new 
   FROM u_users 
   WHERE timezone != timezone_new;
   
   -- Drop old column and rename new one
   ALTER TABLE u_users DROP COLUMN timezone;
   ALTER TABLE u_users RENAME COLUMN timezone_new TO timezone;

3. Code changes:

   • Remove $tz_to_name mapping array
   • Update timezone selection UI to use standard IANA identifiers
   • Add timezone validation using DateTimeZone::listIdentifiers()
   • Update timezone offset calculation to handle historical changes

4. UI improvements:

   • Group timezones by region
   • Show current offset and DST status
   • Add search/filter functionality
   • Display friendly names (e.g., "Pacific Time (US & Canada)")