Add Linux support: system battery, Bluetooth and USB HID devices

[PyFlat-JR]

What does this PR do?

Adds Linux x64 as a third platform next to Windows and macOS, and releases it as 1.5.0.

Battery sources on Linux

• This computer: read from /sys/class/power_supply. Two batteries are reported as one level,
  and a battery held below full by a charge threshold reports an unknown charging state.
• Bluetooth devices: BlueZ's Battery1 over D-Bus (through busctl, so a systemd-based
  distribution is needed). Only connected devices are listed, under the name the desktop shows. The
  Bluetooth snapshot logic is now shared with the macOS reader.
• USB HID devices from the catalog: read through hidraw ioctls (LinuxHidraw).
• Android phone: unchanged, through Macro Deck's adb.

HID access

• Only root can open /dev/hidraw* by default. packaging/linux/70-device-battery-info.rules grants
  the logged-in user access to each supported product id only, never a whole vendor.
• docs/linux-setup.md prints the rule line by line instead of downloading it
  as root. A test keeps the rule and the guide in sync.
• When Linux refuses to open a device, Macro Deck shows an integration issue on the plugin that names
  the device and links to the guide.

Build and release

• manifest.json gets a linux-x64 entrypoint, and CI now also runs on ubuntu-latest.
• make pack packs linux-x64 on Linux.
• make release without VERSION now tags the version already in manifest.json. Both forms refuse
  a version that isn't newer than the latest release tag.

Docs

• README, AGENTS.md, CONTRIBUTING.md and docs/adding-a-device.md cover Linux.
• Links now point to docs.macro-deck.app and the moved release page.

Checklist

• I have read AGENTS.md and CONTRIBUTING.md.
• dotnet build and dotnet test pass locally (0 warnings, 251 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)

Linux: a Razer DeathAdder V3 Pro over the dongle and the cable. The other catalog devices have not been
tried on Linux yet.