Installation script fails with tar/gzip errors

[iAziz786]

The install.sh script fails during installation with the following errors:

Checking for existing installation...
Cleanup complete.
  % Total    Received % Xferd  Average Speed   Time     Time     Time Current                                Dload  Upload  Spent  Left  Pending
100     9  100     9  0     0     26      0 --:-- --:-- -- -- -- -- --    26
tar: This does not look like a tar archive
gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now
Moving binary to /usr/bin, password prompt might be required.
mv: cannot stat './hostlink': No such file or directory
Creating hostlink directories...
Directories created.
Creating environment configuration...
Environment file created at /etc/hostlink/hostlink.env
Installing systemd service...
cp: cannot stat './scripts/hostlink.service': No such file or directory
Failed to enable unit: Unit file hostlink.service does not exist.
Failed to start hostlink.service: Unit file hostlink.service not found.
Service started.
``"

## Root Cause Analysis

1. **Download/Extraction Failure**: The tar extraction fails because the downloaded file is only 9 bytes (just "Not Found" text), indicating the download URL returns a 404 or the version doesn't exist.

2. **Relative Path Issue**: Even if the download succeeds, the install script uses relative paths like `./scripts/hostlink.service` and `./hostlink`. When run after extracting the tar.gz, these paths may not resolve correctly depending on the working directory.

## Expected Behavior

- Installation should either succeed or provide clear error messages about what's failing
- Download URLs should be validated before attempting extraction
- The script should handle relative paths correctly

## Environment

- Linux (x86_64/arm64)
- Installation method: `curl -L -o install.sh https://raw.githubusercontent.com/selfhost-dev/hostlink/main/scripts/linux/install.sh && sudo bash install.sh` (or similar)

## Suggested Fixes

1. Add error handling for the download step to verify the file is a valid tar.gz before extraction
2. Use absolute paths based on the script's directory (`/bin`) instead of relative paths
3. Add validation that the download succeeded (check HTTP status code and file size)
4. Add a helpful error message if the version doesn't exist or download fails