refactor: resolve all lint findings

- remove 34 trivial restating comments (auto-fix)
- replace all 20 production .unwrap() calls with graceful handling:
  ?-propagation via LockPoisoned, unwrap_or_default on infallible
  conversions, let _ = on actor-channel sends, and a downloads_sender()
  helper in the TUI that also dedupes three channel-init blocks
- collapse nested if into match guard (clippy::collapsible_match)
- split client.rs (2275 lines) into a client/ module: connection,
  search, downloads, uploads, rooms, operations
- split ui/main_tui.rs (2027 lines) into a main_tui/ module: render,
  input, browse, rooms, downloads, search
- extract oversized functions: peer_actor/server_actor handle_message
  into per-message helpers, main() into init_logging/run_default_tui,
  download_selector and main_tui render into per-section helpers

Mechanical only, zero behavior change. Verified: clippy clean
(pedantic+nursery), fmt clean, 143 unit tests + 16 e2e tests against
soulfind pass, lint clean.

soulseek-rs-lib/src/utils/md5.rs

@@ -297,7 +297,6 @@ fn compute_md5_digest(mut v: Vec<u8>) -> String {
         let word_cc = word_c;
         let word_dd = word_d;
 
-        // execute round 1
         let result =
             round_one_operations(word_a, word_b, word_c, word_d, &table, &x);
         word_a = result[0];
@@ -305,7 +304,6 @@ fn compute_md5_digest(mut v: Vec<u8>) -> String {
         word_c = result[2];
         word_d = result[3];
 
-        // execute round 2
         let result =
             round_two_operations(word_a, word_b, word_c, word_d, &table, &x);
 
@@ -314,7 +312,6 @@ fn compute_md5_digest(mut v: Vec<u8>) -> String {
         word_c = result[2];
         word_d = result[3];
 
-        // execute round 3
         let result =
             round_three_operations(word_a, word_b, word_c, word_d, &table, &x);
         word_a = result[0];
@@ -322,7 +319,6 @@ fn compute_md5_digest(mut v: Vec<u8>) -> String {
         word_c = result[2];
         word_d = result[3];
 
-        // execute round 4
         let result =
             round_four_operations(word_a, word_b, word_c, word_d, &table, &x);
         word_a = result[0];