refactor(viewer): explorer UX cleanups (menu, loading, tree, status bar)

Four small UX fixes to the asset explorer:

1. Remove the no-op "View" menu (Textures/Meshes/Audio toggles + Wireframe
   checkbox) — none were wired to anything. Dropped ViewMode + the current_mode/
   wireframe fields from ViewerState (nothing read them); the central panel
   already dispatches on which preview resource is populated.

2. Loading indicator — clicking a drawer item now flips FileBrowserState.loading
   and the central panel shows a centered spinner + "Loading <name>…" until the
   background read/decode applies, instead of leaving the previous item on screen
   with no feedback. Cleared in poll_loader_channel on FileLoaded/PakLoaded/
   Error/Cancelled.

3. Drawer shows only the .pak index, not its .pNN data segments — the segments
   are the pack's body (loaded when the .pak is opened), so listing them (with no
   preview) was noise. Dropped the virtual "<stem>.pak" folder too; the .pak now
   sits as a normal leaf in its real directory.

4. Bottom status bar no longer overflows — horizontal_wrapped, only the source's
   final path component (full path on hover), file count, and a right-aligned,
   shortened controls hint. Removed the mode label.

Viewer + formats build; all tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/sylpheed-viewer/src/lib.rs

@@ -28,36 +28,16 @@ pub mod ui;
 /// Global viewer state, shared across all UI and rendering systems.
 #[derive(Resource)]
 pub struct ViewerState {
-    /// Which type of asset is currently being browsed / previewed.
-    pub current_mode: ViewMode,
-    /// Whether to render meshes in wireframe mode (toggle with F2).
-    pub wireframe: bool,
     /// Whether the plain-text viewer wraps long lines.
     pub text_wrap: bool,
 }
 
 impl Default for ViewerState {
     fn default() -> Self {
-        Self {
-            current_mode: ViewMode::Textures,
-            wireframe: false,
-            text_wrap: true,
-        }
+        Self { text_wrap: true }
     }
 }
 
-/// The active view / asset type being inspected.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
-pub enum ViewMode {
-    /// XPR2 texture preview (Milestone 1).
-    #[default]
-    Textures,
-    /// 3D mesh viewer (Milestone 2+).
-    Mesh,
-    /// Audio waveform / playback (Milestone 2+).
-    Audio,
-}
-
 // ── App builder ──────────────────────────────────────────────────────────────
 
 /// Build and run the viewer application.