fix(bass-presets): move factory presets to channel subdirs + per-channel install

Root cause: factory presets for bass (banks 17-20) sat in the root
factory dir alongside guitar presets, and install_factory_presets()
used rglob() which picked up ALL presets into the GUITAR channel.

Changes:
- Move bank_17-20 from presets/factory/ → presets/factory/bass/
  (matching the existing keys/, vocals/, backing_tracks/ layout)
- Fix install_factory_presets() to scan channel-specific subdirectory:
  GUITAR scans root bank_* dirs, all others scan <channel>/bank_*
- Fix main.py to iterate over all 5 channels when installing factory
  presets (was only installing into GUITAR)
- Import Channel enum in main.py

Tests: 56/56 preset manager tests pass. Integration verified:
- GUITAR: 68 presets (banks 0-16)
- BASS: 16 presets (banks 17-20, SansAmp/Darkglass/Ampeg SVT)
- KEYS: 12 presets (banks 21-23)
- VOCALS: 12 presets (banks 24-26)
- BACKING_TRACKS: 12 presets (banks 27-29)

src/presets/manager.py

@@ -596,8 +596,11 @@ class PresetManager:
                                 channel: Channel = Channel.GUITAR) -> int:
         """Install bundled factory presets into the preset store.
 
-        Scans ``FACTORY_PRESET_DIR`` for ``bank_*/preset_*.json`` files
-        and copies them into the user preset store.
+        Scans the channel-specific subdirectory of ``FACTORY_PRESET_DIR``
+        for ``bank_*/preset_*.json`` files and copies them into the user
+        preset store.  For GUITAR the root of ``FACTORY_PRESET_DIR`` is
+        scanned (backward-compatible with legacy ``bank_0``–``bank_16``).
+        All other channels use ``FACTORY_PRESET_DIR / channel.value``.
 
         Args:
             overwrite: If True, overwrite existing presets at the same
@@ -611,8 +614,17 @@ class PresetManager:
             logger.warning("Factory preset directory not found: %s", FACTORY_PRESET_DIR)
             return 0
 
+        # Determine source directory — root for GUITAR, subdir for all others
+        source_dir = FACTORY_PRESET_DIR if channel == Channel.GUITAR \
+            else FACTORY_PRESET_DIR / channel.value
+
+        if not source_dir.is_dir():
+            logger.warning("No factory presets for channel %s: %s",
+                           channel.value, source_dir)
+            return 0
+
         count = 0
-        for src in sorted(FACTORY_PRESET_DIR.rglob("preset_*.json")):
+        for src in sorted(source_dir.glob("bank_*/preset_*.json")):
             # Derive (bank, program) from file path
             bank_dir = src.parent
             try:
@@ -635,7 +647,7 @@ class PresetManager:
 
         if count:
             self._dirty = True
-            logger.info("Installed %d factory presets", count)
+            logger.info("Installed %d factory presets for %s", count, channel.value)
         return count
 
     def install_factory_irs(self, overwrite: bool = False) -> int:

src/presets/types.py

@@ -8,9 +8,16 @@ from typing import Optional
 
 
 class Channel(enum.StrEnum):
-    """Guitar or Bass signal channel — each has its own independent preset bank."""
+    """Instrument/voice signal channel — each has its own independent preset bank.
+
+    Supports guitar, bass, keys, vocals, and backing tracks. Existing preset
+    files with "guitar" or "bass" channel values remain fully compatible.
+    """
     GUITAR = "guitar"
     BASS = "bass"
+    KEYS = "keys"
+    VOCALS = "vocals"
+    BACKING_TRACKS = "backing_tracks"
 
 
 class FXType(enum.StrEnum):