Workstream C: convert commands/download to a folder module, extract pinning and smart-cache commands

- Move commands/download.rs to commands/download/mod.rs.
- Extract pin/unpin/is_pinned into download/pinning.rs.
- Extract smart-cache stats/config + album recommendation commands into
  download/smart_cache.rs.
- Re-exported via pub use so command names stay at commands::download::*;
  invoke_handler unchanged, all tests pass.

src-tauri/src/utils/conversions.rs

@@ -24,6 +24,7 @@ pub const TICKS_PER_SECOND: i64 = 10_000_000;
 ///
 /// # Example
 /// ```
+/// # use jellytau_lib::utils::conversions::*;
 /// let ticks = seconds_to_ticks(1.5); // 15,000,000 ticks
 /// ```
 #[inline]
@@ -41,6 +42,7 @@ pub fn seconds_to_ticks(seconds: f64) -> i64 {
 ///
 /// # Example
 /// ```
+/// # use jellytau_lib::utils::conversions::*;
 /// let seconds = ticks_to_seconds(15_000_000); // 1.5 seconds
 /// ```
 #[inline]
@@ -61,6 +63,7 @@ pub fn ticks_to_seconds(ticks: i64) -> f64 {
 ///
 /// # Example
 /// ```
+/// # use jellytau_lib::utils::conversions::*;
 /// let normalized = percent_to_volume(75.0); // 0.75
 /// ```
 #[inline]
@@ -81,6 +84,7 @@ pub fn percent_to_volume(percent: f64) -> f64 {
 ///
 /// # Example
 /// ```
+/// # use jellytau_lib::utils::conversions::*;
 /// let percent = volume_to_percent(0.75); // 75.0
 /// ```
 #[inline]
@@ -99,6 +103,7 @@ pub fn volume_to_percent(volume: f64) -> f64 {
 ///
 /// # Example
 /// ```
+/// # use jellytau_lib::utils::conversions::*;
 /// let formatted = format_time(225.0); // "3:45"
 /// ```
 pub fn format_time(seconds: f64) -> String {
@@ -121,6 +126,7 @@ pub fn format_time(seconds: f64) -> String {
 ///
 /// # Example
 /// ```
+/// # use jellytau_lib::utils::conversions::*;
 /// let short = format_time_long(225.0);  // "3:45"
 /// let long = format_time_long(5025.0);  // "1:23:45"
 /// ```
@@ -147,6 +153,7 @@ pub fn format_time_long(seconds: f64) -> String {
 ///
 /// # Example
 /// ```
+/// # use jellytau_lib::utils::conversions::*;
 /// let progress = calculate_progress(45.0, 180.0); // 25.0
 /// ```
 pub fn calculate_progress(position: f64, duration: f64) -> f64 {