[PyFlat-JR]

What does this PR do?

Lets both widgets show devices that other providers publish to Macro Deck's battery catalog (SDK beta.17, IIntegrationContext.Batteries), such as the phones from the built-in ADB integration, next to this plugin's own devices.

• Picker: the widget's device list shows this plugin's devices first, then every catalog device from another provider, labelled device - provider. This plugin's own catalog entries are filtered out by the id it published.
• Rows: a picked external device is drawn like our own devices, with level, charging state, time to full and our icon for its kind. A disconnected device shows as stale.
• Storage: a widget stores an external device by its catalog id (provider:id). The colon keeps those ids from ever clashing with our own device ids.
• Existing widgets: an empty selection still means this plugin's devices only, so existing widgets don't gain rows.
• Widgets only: external devices get no battery_<id>_* variables, no events and no trend.
• Row keys: a widget element key can't contain a colon, so external rows are keyed by a short SHA-256 hash of the catalog id (ExternalBatteryDevices.RowKey). A test opens a real widget session to cover this.
• Live updates: the host drops a context Changed handler on every re-initialization, and widget sessions stay open across that. So the integration re-subscribes in InitializeAsync and raises its own BatteryCatalogChanged, which the sessions listen to.
• Mapping back: headphones become our headset icon, laptop becomes the system icon, and watch and unknown kinds become other. NotCharging reads as Full at 100 % and Unknown below, matching our macOS and Linux sources.
• Docs: README, AGENTS.md and the sourceIds description in the widget data schema are updated. There is one new string, Widgets.Config.Devices.External.

Checklist

• I have read AGENTS.md and CONTRIBUTING.md.
• dotnet build and dotnet test pass locally (0 warnings, 316 tests).
• I updated README.md / AGENTS.md if this changes what they describe.
• If this touches a device backend: I tested it against the real hardware (see below).

Device/hardware tested against (if applicable)

Not a device backend.