feat(viewer): directory-tree browser + plain-text viewer

Explorer drawer was a flat list of all 353 disc files and only textures had a
preview. Two additions:

1. Directory tree — the left drawer is now a collapsible `📁` tree, rebuilt each
   frame from the path list (ui.rs: build_tree / split_for_tree / render_dir).
   Filtering narrows leaves and auto-expands matches; `.p00…p04` data segments
   nest under a virtual `<stem>.pak` node. Selection still keys off the original
   file index, so the loader pipeline is untouched.

2. Plain-text viewer — config.ini and any file that sniffs as text now show
   read-only, selectable, monospace content with a Wrap toggle and an encoding
   label. Detection/decoding live in the Bevy-free formats crate:
   vfs::is_probably_text + vfs::decode_text (BOM-aware UTF-8/UTF-16LE/BE, BOM-less
   UTF-16LE heuristic, NUL-reject + 95%-printable fallback) — pure std, WASM-safe,
   unit-tested (6 new tests), and reused by the CLI sniffer (new cyan `txt`).

New TextPreview resource (registered unconditionally so it exists on wasm);
populated in apply_loaded_texture with a 2 MiB char-boundary-safe cap. Also fixes
a latent bug: switching from a texture to a non-texture left stale preview state —
both texture and text state now reset on every selection.

ViewerState gains text_wrap. Formats tests: 22 pass; workspace compiles.

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

crates/sylpheed-cli/src/main.rs

@@ -272,6 +272,7 @@ fn cmd_sniff(dir: &Path, unknown_only: bool) -> Result<()> {
             "bin" => label.red().to_string(),
             "xpr" => label.green().to_string(),
             "dds" => label.green().to_string(),
+            "txt" => label.cyan().to_string(),
             _ => label.yellow().to_string(),
         };