Architecture remediation A/B/F: poison-tolerant locks, graceful backend init, doc fixes

Workstream A — poison-tolerant locking:
- Add utils/lock.rs with MutexSafe/RwLockSafe extension traits that recover a
  poisoned std::sync lock instead of panicking, plus unit tests.
- Replace all 153 .lock().unwrap() and 4 .read()/.write().unwrap() production
  sites with _safe variants across 14 files, eliminating the player
  crash-cascade class. Tokio async mutexes are unchanged.

Workstream B — graceful backend init:
- create_player_backend no longer panics when MPV/ExoPlayer fail to initialize;
  it falls back to NullBackend and emits a backend-init-failed event so the UI
  can show "playback unavailable" instead of the app crashing. Fatal DB-setup
  panics are kept.

Workstream F — doc reconciliation:
- Rewrite software-architecture.md's inaccurate "thin UI / ~800 lines" claims to
  reflect reality (~20.5k non-test frontend) and document the events+polling
  hybrid plus the new locking/backend-init behavior.

src-tauri/src/playback_mode/mod.rs

@@ -1,3 +1,4 @@
+use crate::utils::lock::{MutexSafe, RwLockSafe};
 use log::{debug, error, info};
 use serde::{Deserialize, Serialize};
 use std::sync::{
@@ -43,13 +44,13 @@ impl PlaybackModeManager {
 
     /// Get current playback mode
     pub fn get_mode(&self) -> PlaybackMode {
-        self.current_mode.read().unwrap().clone()
+        self.current_mode.read_safe().clone()
     }
 
     /// Set playback mode (internal use)
     pub fn set_mode(&self, mode: PlaybackMode) {
         log::info!("[PlaybackMode] Setting mode to: {:?}", mode);
-        let mut current = self.current_mode.write().unwrap();
+        let mut current = self.current_mode.write_safe();
         *current = mode;
     }
 
@@ -193,7 +194,7 @@ impl PlaybackModeManager {
             debug!("[PlaybackMode] Player controller lock acquired");
 
             let queue_arc = player.queue();
-            let queue = queue_arc.lock().unwrap();
+            let queue = queue_arc.lock_safe();
             let state = player.state();
 
             let original_index = queue.current_index().unwrap_or(0);
@@ -399,7 +400,7 @@ impl PlaybackModeManager {
             // Log queue state BEFORE stop
             {
                 let queue_arc = player.queue();
-                let queue = queue_arc.lock().unwrap();
+                let queue = queue_arc.lock_safe();
                 info!(
                     "[PlaybackMode] BEFORE STOP: Queue has {} items, current_index={:?}",
                     queue.items().len(),
@@ -412,7 +413,7 @@ impl PlaybackModeManager {
             // Log queue state AFTER stop (should be unchanged)
             {
                 let queue_arc = player.queue();
-                let queue = queue_arc.lock().unwrap();
+                let queue = queue_arc.lock_safe();
                 info!(
                     "[PlaybackMode] AFTER STOP: Queue has {} items, current_index={:?}",
                     queue.items().len(),