✨ feat: periodic update check on startup

On startup, if `update_check_interval_hours` have elapsed since the last
check, fetches the latest release tag from the Gitea API and prints a
notice when a newer version is available. Silent on network failure or
no update found.

- New `update_check` module with injected fetcher for full testability
- Cache at ~/.cache/tmuxido/update_check.json tracks timestamp + version
- `fetch_latest_tag` and `version_compare` promoted to `pub(crate)`
- 6 unit tests covering disabled, interval not elapsed, fetch triggered,
  equal versions, update available, and current-newer edge case

src/self_update.rs

@@ -27,7 +27,7 @@ fn detect_arch() -> Result<&'static str> {
 }
 
 /// Fetch latest release tag from Gitea API
-fn fetch_latest_tag() -> Result<String> {
+pub(crate) fn fetch_latest_tag() -> Result<String> {
     let url = format!("{}/api/v1/repos/{}/releases?limit=1&page=1", BASE_URL, REPO);
 
     let output = Command::new("curl")
@@ -166,7 +166,7 @@ pub fn self_update() -> Result<()> {
 }
 
 /// Compare two semver versions
-fn version_compare(a: &str, b: &str) -> std::cmp::Ordering {
+pub(crate) fn version_compare(a: &str, b: &str) -> std::cmp::Ordering {
     let parse = |s: &str| {
         s.split('.')
             .filter_map(|n| n.parse::<u32>().ok())