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)

main.py

@@ -28,7 +28,7 @@ from src.dsp.pipeline import AudioPipeline
 from src.dsp.nam_host import NAMHost
 from src.dsp.ir_loader import IRLoader
 from src.presets.manager import PresetManager
-from src.presets.types import MIDIMapping, Preset
+from src.presets.types import Channel, MIDIMapping, Preset
 from src.midi.handler import MIDIHandler
 from src.ui.footswitch import FootswitchController, FootSwitch, SwitchAction
 from src.ui.leds import LEDController, LEDDriver, LEDConfig, LEDPattern
@@ -170,11 +170,15 @@ class PedalApp:
                 audio_pipeline=self.pipeline,
             )
 
-            # Install factory presets (non-destructive by default)
+            # Install factory presets for all channels (non-destructive by default)
             if pcfg.get("install_factory", True):
-                installed = self.presets.install_factory_presets(overwrite=False)
-                if installed:
-                    logger.info("Installed %d factory preset(s)", installed)
+                for ch in Channel:
+                    installed = self.presets.install_factory_presets(
+                        overwrite=False, channel=ch,
+                    )
+                    if installed:
+                        logger.info("Installed %d factory preset(s) for %s",
+                                    installed, ch.value)
                 ir_installed = self.presets.install_factory_irs(overwrite=False)
                 if ir_installed:
                     logger.info("Installed %d factory cab IR(s)", ir_installed)