chore(lint): clean up clippy warnings across workspace

Auto-applied cargo clippy --fix across 11 files — needless return,
unnecessary cast, map_or simplification, repeat().take() → repeat_n(),
iter().any() → contains(), manual char comparison, lifetime elision,
push_str single-char, reference immediately dereferenced.

Also fixed three lints on file_storage.rs manually: two doc-list-item
overindentations, plus the same needless-return. Baseline main was
not clippy-clean with -D warnings before; after this pass one
needless_range_loop warning remains (live.rs:1089) that clippy's
suggested rewrite would make less readable — left for a dedicated
refactor session.

Build + workspace tests remain green (245 passing, 0 failing, 1
ignored).

src-tauri/src/commands/diagnostics.rs

@@ -247,7 +247,7 @@ pub async fn generate_diagnostic_report(
         .map(|d| d.as_secs())
         .unwrap_or(0);
     out.push_str(&format!("- Generated: unix `{}`\n", now));
-    out.push_str("\n");
+    out.push('\n');
     out.push_str(
         "> This report is local-only until you choose to share it. \
                   Review the contents below before sending to anyone.\n\n",