security, refactor: resolve critical namespace escapes, path traversal, concurrency races, and resource leaks

This commit addresses several security vulnerabilities, undefined behaviors, race conditions, and resource leaks across the application:

1. Path Traversal & Arbitrary File/Directory Actions:
   - Implemented `IsValidProfileName` in `internal/cli/cli.go` to restrict profile names to alphanumeric characters, dashes, and underscores.
   - Applied validation to all CLI paths (`--profile`, `import`, `configure`, `delete`, `stop`) to prevent directory traversal and unauthorized directory or file creations/deletions.
   - Added `TestIsValidProfileName` in `internal/cli/cli_test.go`.

2. Network Namespace Escape via Compromised Thread recycling:
   - Fixed `HostBind.Open` in `internal/wireguard/wireguard.go` to panic immediately instead of returning an error if restoring the isolated namespace fails. This prevents Go from returning the compromised thread (still in host namespace) to the runtime pool.

3. Concurrency Race Conditions & Thread Migration:
   - Added `runtime.LockOSThread()` in `JoinExistingNamespace` (`internal/namespace/pinning.go`) to ensure the goroutine stays on the modified namespace thread before executing the command.
   - Implemented profile locking using advisory file locks (`unix.Flock`) on a `.lock` file in the user's runtime directory (with platform stubs in `internal/namespace/lock_linux.go` and `internal/namespace/lock_stub.go`).
   - Integrated locking during `App.Run` and `App.ExecuteCommand`, releasing the lock right before spawning the wrapped process.

4. File Descriptor Leaks on Bootstrap Failures:
   - Refactored `Bootstrap()` in `internal/namespace/namespace.go` to use named return values and a deferred cleanup loop that closes `execFd`, `hostNetFd`, and the duplicated `hostSocketFd` if `syscall.Exec` fails.
   - Added an explicit `conn.Close()` on the original socket connection after duplication.

5. Glibc Undefined Behavior / Crash on argc == 0:
   - Corrected `internal/namespace/launcher_src/launcher.c` to not reference `argv[0]` when `argc < 1`. Recompiled `internal/namespace/launcher.bin`.

6. DNS Fallback Usability & Import Safety:
   - Added validation in `ExecuteCommand` to issue a warning when falling back to `1.1.1.1` if the configuration does not route all traffic (`0.0.0.0/0` or `::/0`).
   - Prevented silent overwrites in `handleProfileImport` if the destination profile already exists, and added a corresponding unit test verifying failure.

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