refactor: eliminate CHOICE variables and centralize configuration (#1427)

## Summary

- Eliminate all CHOICE intermediary variables (~17 variables removed)
- Replace numeric values with descriptive string values throughout
- Centralize configuration defaults in a single function
- Add comprehensive validation for all configuration values
- Add reusable helper functions for interactive prompts
- Fix non-interactive mode to skip interactive prompts entirely

## Changes

### Configuration Values Now Use Strings

| Variable | Before | After |
|----------|--------|-------|
| TLS_SIG | 1, 2, 3 | "crypt-v2", "crypt", "auth" |
| CERT_TYPE | 1, 2 | "ecdsa", "rsa" |
| DNS | 1-13 | "cloudflare", "quad9", "custom", etc. |

### Code Reduction

- Removed ~80 lines of duplicate defaults from `installOpenVPN()`
- Removed ~60 lines of CHOICE variable assignments from `cmd_install()`
- Net change: cleaner structure with better separation of concerns

### New Functions

- `set_installation_defaults()`: Single source of truth for all defaults
- `validate_configuration()`: Validates all config values after defaults
applied
- `select_from_array()`: Generic menu selection helper
- `select_with_labels()`: Menu with display labels different from values
- `prompt_yes_no()`: Yes/no prompts with validation
- `prompt_validated()`: Custom value prompts with validation function
- `detect_server_ips()`: Detects server IPv4/IPv6 addresses
- `prepare_network_config()`: Calculates derived network config
(gateways, etc.)

### Configuration Constants

Added readonly arrays for valid options:
- `PROTOCOLS`, `DNS_PROVIDERS`, `CIPHERS`, `CERT_TYPES`
- `CERT_CURVES`, `RSA_KEY_SIZES`, `TLS_VERSIONS`
- `TLS13_CIPHERSUITES_OPTIONS`, `TLS_GROUPS_OPTIONS`
- `HMAC_ALGORITHMS`, `TLS_SIG_MODES`

### Non-Interactive Mode Fix

- `installQuestions()` is now only called in interactive mode
- IP detection and gateway calculations extracted to separate functions
- Renamed `AUTO_INSTALL` to `NON_INTERACTIVE_INSTALL` for clarity
- Non-interactive installs no longer hang waiting for user input

## Test plan

- [ ] Run `make test` (default Ubuntu)
- [ ] Test interactive installation
- [ ] Test non-interactive installation with CLI flags
- [ ] Test non-interactive installation with environment variables
- [ ] Test invalid configuration values are rejected

.github/workflows/lint.yml

@@ -23,6 +23,6 @@ jobs:
           fetch-depth: 0
           persist-credentials: false
       - name: Lint Code Base
-        uses: super-linter/super-linter@502f4fe48a81a392756e173e39a861f8c8efe056 # v8
+        uses: super-linter/super-linter@502f4fe48a81a392756e173e39a861f8c8efe056 # v8.3.0
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}