feat(providers): add a pluggable station-catalogue seam

The app now holds one RadioProvider and asks it for stations, song history and stream URLs instead of importing SomaFM's HTTP client directly.

- models.py: provider-neutral Song/Station plus the shared parsers every catalogue needs (Station.provider/.uid keep ids unique once there is more than one).
- providers/base.py: the RadioProvider interface and the toolkit it needs in practice - httpx client factory, .pls/.m3u resolution with an 8h disk cache, and a default stream_candidates() strategy.
- providers/somafm/: everything SomaFM-specific (channels.json, songs/<id>.json, PLS layout, guessed icecast mirrors) behind SomaFMProvider.
- providers/__init__.py: registry, get_provider() and --provider, remembered in the config file.

Widgets render branding, splash copy and the help credits from the active provider, so a second catalogue needs no UI changes; with SomaFM selected the screen is byte-identical to before.

beefy_player/widgets/help.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import sys
 import textwrap
+from typing import TYPE_CHECKING, cast
 
 from rich.console import Group
 from rich.table import Table
@@ -14,6 +15,9 @@ from textual.widgets import Static
 
 from .common import SPECTRUM_STOPS, gradient
 
+if TYPE_CHECKING:
+    from ..app import BeefyPlayerApp
+
 HELP_ROWS: tuple[tuple[str, str], ...] = (
     ("↑ / ↓  j / k", "move through the station list"),
     ("enter", "tune in to the selected station"),
@@ -35,13 +39,6 @@ HELP_ROWS: tuple[tuple[str, str], ...] = (
     ("q", "quit"),
 )
 
-ABOUT_COMMON = (
-    (
-        "Stations and song history come from somafm.com "
-        "(channels.json + songs/<station>.json)."
-    ),
-)
-
 ABOUT_LINUX = (
     (
         "The spectrum captures your actual PulseAudio/PipeWire output with "
@@ -50,7 +47,7 @@ ABOUT_LINUX = (
     ),
     (
         "The desktop picks us up over MPRIS as a "
-        "somafmtui media player — try `playerctl -l` / `playerctl metadata`."
+        "beefyplayer media player — try `playerctl -l` / `playerctl metadata`."
     ),
 )
 
@@ -68,11 +65,15 @@ ABOUT_MACOS = (
     ),
 )
 
-ABOUT = ABOUT_COMMON + (ABOUT_MACOS if sys.platform == "darwin" else ABOUT_LINUX)
-
 
 class HelpScreen(ModalScreen[None]):
     def compose(self) -> ComposeResult:
+        provider = cast("BeefyPlayerApp", self.app).provider
+        about = (
+            f"Stations and song history come from {provider.name} "
+            f"({provider.station_source} + {provider.history_source}).",
+        ) + (ABOUT_MACOS if sys.platform == "darwin" else ABOUT_LINUX)
+
         table = Table.grid(padding=(0, 2), expand=False)
         table.add_column(style="bold cyan", no_wrap=True, justify="right")
         table.add_column(style="")
@@ -81,11 +82,11 @@ class HelpScreen(ModalScreen[None]):
 
         header = Text()
         header.append("◤ ", style=f"bold {gradient(SPECTRUM_STOPS, 0.9)}")
-        header.append("SOMAFM TUI", style=f"bold {gradient(SPECTRUM_STOPS, 0.55)}")
+        header.append("BEEFY PLAYER", style=f"bold {gradient(SPECTRUM_STOPS, 0.55)}")
         header.append("  keyboard & credits", style="dim")
 
         footer = Text()
-        for paragraph in ABOUT:
+        for paragraph in about:
             for line in textwrap.wrap(paragraph, width=70):
                 footer.append(line + "\n", style="dim")