Daemon fails to start (and may hang on exit) when IPv6 is disabled in kernel: fatal sysctl on net.ipv6.conf.dae0.*

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### Checks

- [x] I have searched the existing issues
- [x] I have read the documentation
- [ ] Is it your first time sumbitting an issue

### Current Behavior

When IPv6 is disabled at the kernel level (e.g., ipv6.disable=1 or IPv6 module not available), dae fails during netns setup because it unconditionally writes IPv6 sysctls for the host veth (dae0).​
This triggers an error like “failed to set disable_ipv6 for dae0: no such file or directory” and then causes a chain reaction where the process cannot exit cleanly (appears to hang / not terminate as expected).
At control/netns_utils.go around line 289 (https://github.com/daeuniverse/dae/blob/main/control/netns_utils.go#L289), dae writes IPv6 sysctls (e.g., net.ipv6.conf.dae0.disable_ipv6 / forwarding) unconditionally before continuing netns setup.​
If IPv6 is disabled at the kernel level, the IPv6 sysctl tree (e.g., /proc/sys/net/ipv6/...) may not exist, so this operation can return ENOENT, and crashing/failing hard at that point is incorrect behavior.​
Please check whether IPv6 is available/enabled before touching any net.ipv6.* sysctls (or treat ENOENT as “IPv6 unsupported” and skip IPv6 initialization), so dae can still run in IPv4-only systems and exit cleanly on other errors.

### Expected Behavior

_No response_

### Steps to Reproduce

1. add kernel arg ipv6.disable=1

### Environment

- **Dae version (use `dae --version`)**: daed version v1.21.1
- **OS (e.g `cat /etc/os-release`)**: ubuntu 24.04
- **Kernel (e.g. `uname -a`)**: linux 6.x
- **Others**:

### Anything else?

_No response_

Start in control/netns_utils.go around line 289, where dae writes the net.ipv6.* sysctls during network namespace setup. Reproduce with the ipv6.disable=1 kernel argument and inspect the ENOENT path. Done means dae can run on an IPv4-only system and exits cleanly when IPv6 sysctls are unavailable.

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