Workstream E (backend): wire tauri-specta — annotate commands, derive Type, generate-ready Builder

- Add #[specta::specta] to all 201 #[tauri::command] functions.
- Derive specta::Type on all IPC DTOs (repository/types, settings, player/storage/
  download command DTOs, player enums, jellyfin SessionInfo/NowPlayingItem/PlayState,
  ThumbnailCacheStats, DownloadInfo, CacheConfig, etc.).
- Replace tauri::generate_handler! with a tauri_specta::Builder + collect_commands!
  in lib.rs (exports bindings.ts in debug builds).

Two contract changes required by specta constraints (frontend migration follows):
- specta caps command arity at 10 args: download_item_and_start / download_item /
  download_video now take a single request struct (params bundled, body unchanged
  via destructuring).
- specta can't parse split serde rename_all: SessionInfo/NowPlayingItem/PlayState
  switched to rename_all = "PascalCase" (Jellyfin deserialization preserved; these
  now serialize PascalCase to the frontend).

cargo check --lib is clean (0 errors). Frontend migration to bindings.ts is the next step.

src-tauri/src/commands/conversions.rs

@@ -16,6 +16,7 @@ use crate::utils::conversions::{
 /// # Returns
 /// Formatted string like "3:45" or "12:09"
 #[tauri::command]
+#[specta::specta]
 pub fn format_time_seconds(seconds: f64) -> String {
     format_time(seconds)
 }
@@ -32,6 +33,7 @@ pub fn format_time_seconds(seconds: f64) -> String {
 /// # Returns
 /// Formatted string like "1:23:45" or "3:45"
 #[tauri::command]
+#[specta::specta]
 pub fn format_time_seconds_long(seconds: f64) -> String {
     format_time_long(seconds)
 }
@@ -44,6 +46,7 @@ pub fn format_time_seconds_long(seconds: f64) -> String {
 /// # Returns
 /// Time in seconds
 #[tauri::command]
+#[specta::specta]
 pub fn convert_ticks_to_seconds(ticks: i64) -> f64 {
     ticks_to_seconds(ticks)
 }
@@ -57,6 +60,7 @@ pub fn convert_ticks_to_seconds(ticks: i64) -> f64 {
 /// # Returns
 /// Progress as percentage (0.0 to 100.0)
 #[tauri::command]
+#[specta::specta]
 pub fn calc_progress(position: f64, duration: f64) -> f64 {
     calculate_progress(position, duration)
 }
@@ -69,6 +73,7 @@ pub fn calc_progress(position: f64, duration: f64) -> f64 {
 /// # Returns
 /// Normalized volume (0.0 to 1.0)
 #[tauri::command]
+#[specta::specta]
 pub fn convert_percent_to_volume(percent: f64) -> f64 {
     percent_to_volume(percent)
 }