Fix user permissions - implement group-based multi-user access

Problem: Users could authenticate but got "permission denied" when creating files
Root cause: Shares used force_user which prevented proper multi-user access

Solution: Group-based permission system
- Created `sambashare` group for all Samba users
- Users are automatically added to this group on configuration
- Share directories get proper group ownership (chown :sambashare)
- SetGID bit ensures new files inherit group ownership
- Removed force_user directive for multi-user shares
- All shares now use force_group=sambashare

Changes in configure_user_shares():
- Creates sambashare group if it doesn't exist
- Adds user to sambashare group
- Sets filesystem permissions: chown -R :sambashare + chmod g+rw
- Applies SetGID bit: chmod g+s
- Removes force_user from share config
- Sets force_group=sambashare

Changes in disk share creation:
- Creates sambashare group automatically
- Adds detected user to sambashare
- Sets proper filesystem permissions on mount
- Uses force_group=sambashare instead of user's primary group
- Removed force_user directive entirely

Benefits:
✅ Multiple users can read/write files without permission errors
✅ Files preserve actual creator ownership (not forced)
✅ Proper Unix permissions maintained
✅ Works across reboots (group membership persists)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

README.md

@@ -101,7 +101,11 @@ sudo ./spravuj_sdileni.sh user-list
 ```bash
 sudo ./spravuj_sdileni.sh user-access [username]
 ```
-Interactively select which shares the user can access.
+Interactively select which shares the user can access. This command:
+- Adds the user to the `sambashare` group
+- Sets proper filesystem permissions (group ownership + read/write)
+- Removes single-user restrictions (`force user`) from shares
+- Enables multi-user access with proper file ownership
 
 **Revoke user access from a share**:
 ```bash
@@ -114,6 +118,21 @@ sudo ./spravuj_sdileni.sh user-delete [username]
 ```
 Optionally removes the system user as well.
 
+### How Permissions Work
+
+FSA uses a group-based permission system for secure multi-user access:
+
+1. **sambashare Group**: All Samba users are added to this group
+2. **Filesystem Permissions**: Share directories are owned by group `sambashare` with read/write access
+3. **SetGID Bit**: Ensures new files inherit the correct group ownership
+4. **No Force User**: Multi-user shares don't force a specific user, preserving actual file ownership
+
+This means:
+- ✅ Multiple users can read/write files
+- ✅ Files show the actual creator's ownership
+- ✅ Proper Unix permissions are maintained
+- ✅ No permission denied errors
+
 ## Share Types
 
 The script creates different types of shares: