Rework connected indicator: filled background instead of dimming

[MegaManSec]

Builds on MegaManSec#104 (thanks!) with a few adjustments:

• Show the connected state by knocking the glyph out of a filled rounded square (composited in code from the existing asset); disconnected keeps the normal icon. Dimming via appearsDisabled made the idle icon read as "app disabled" on whichever Mac doesn't currently hold the peripherals.
• Tri-state presence: no registered peripherals (or Bluetooth state still unknown at launch) shows the plain icon and plain tooltip instead of "Disconnected".
• Tooltips spell out the state: "peripherals connected to this Mac" / "no peripherals connected to this Mac".
• Icon refresh gets its own merged $peripherals/$connectionStates subscription; the transfer-settle observer is back to single-purpose.
• Reverts the #if canImport(PreviewsMacros) guards — that condition is false under Xcode's toolchain too, so previews would compile out entirely. (It looks like a workaround for building with Command Line Tools swiftc, which lacks the previews macro plugin; Xcode builds work with plain #Preview.) Also reverts the .gitignore change.