fix: audit LOW fixes — dead uid, home_dir warn, clippy sweep, debug value (v0.8.5)

- users::User: drop the unused `uid` field and its getuid() assignment.
  The compiler dead_code warning is gone, and the synthetic `u32::MAX`
  sentinel in the panel fallback is obsolete too.
- panel: surface a log::warn! when dirs::home_dir() returns None instead
  of silently falling back to an empty PathBuf that would make avatars
  look for .face in the current working directory.
- Apply three clippy suggestions: two collapsible if-let + && chains in
  users::get_avatar_path_with and config::resolve_background_path_with,
  and a redundant closure in panel::execute_action's spawn_blocking.
- main: require MOONSET_DEBUG=1 to escalate log verbosity — mere
  presence of the var must not dump path info into the journal.

DECISIONS.md

@@ -2,6 +2,13 @@
 
 Architectural and design decisions for Moonset, in reverse chronological order.
 
+## 2026-04-24 – Audit LOW fixes: dead uid field, home_dir warn, clippy sweep, debug value (v0.8.5)
+
+- **Who**: ClaudeCode, Dom
+- **Why**: Five LOW findings cleared in one pass. (1) `User::uid` was populated from `getuid()` but never read — a compiler `dead_code` warning for a field on the public API. (2) Falling back to a synthetic user when `get_current_user()` returned None used `uid: u32::MAX`, an undocumented sentinel that became moot once uid was removed. (3) `dirs::home_dir().unwrap_or_default()` silently yielded `PathBuf::new()` on failure; avatars would then look for `.face` in the current working directory. (4) `cargo clippy` flagged three suggestions (two collapsible `if`, one redundant closure) that had crept in. (5) `MOONSET_DEBUG` promoted log verbosity on mere presence, leaking path information into the journal.
+- **Tradeoffs**: Dropping `uid` from `User` is a minor API break for any internal caller expecting the field — none existed. The synthetic fallback now surfaces `log::warn!` when home resolution fails, which should be rare outside of pathological sandbox environments.
+- **How**: (1) Remove `pub uid: u32` from `User` and the `uid: uid.as_raw()` assignment in `get_current_user`. (2) Panel fallback drops the `uid` field entirely. (3) `dirs::home_dir().unwrap_or_else(|| { log::warn!(...); PathBuf::new() })`. (4) `cargo clippy --fix` for the two collapsible ifs, manual collapse of `if-let` + `&&` chain, redundant closure replaced with the function itself. (5) `MOONSET_DEBUG` now requires the literal value `"1"` to escalate to Debug.
+
 ## 2026-04-24 – Audit MEDIUM fixes: timeout guard, POSIX locale, button desensitize, wallpaper allowlist (v0.8.4)
 
 - **Who**: ClaudeCode, Dom