Isolate XPCHelperClient to the main actor

[Dbhardwaj99]

Second step of the Swift 6 migration tracked in #1054, following #1430.

The problem

XPCHelperClient kept its connection state (remoteService, connection, lunarListener,
monitoringTask) as mutable properties on a non-isolated class, while every method reached that
state through await MainActor.run { ensureRemoteService() } and then used the returned service
outside the actor.

That sent two things across an isolation boundary on every call, which is where 56 of the
project's strict-concurrency diagnostics came from:

Count	Diagnostic
19	sending 'self' risks causing data races
19	capture of 'self' with non-Sendable type 'XPCHelperClient' in a '@Sendable' closure
15	type 'RemoteXPCService<any BoringNotchXPCHelperProtocol>' does not conform to 'Sendable'
2	passing closure as a 'sending' parameter
1	static property 'shared' is not concurrency-safe

The change

Isolating the class to the main actor removes the boundary instead of annotating each crossing.
The MainActor.run hops go away, so neither self nor the service ever leaves the actor, and the
type becomes implicitly Sendable.

• ensureRemoteService() is called directly rather than through MainActor.run
• startMonitoringAccessibilityAuthorization uses Task instead of Task.detached so it stays
  on the actor
• shared and init remain nonisolated, which is safe now that the type is Sendable. This
  keeps the existing call contract, so non-isolated callers such as MediaKeyInterceptor compile
  unchanged
• requestAccessibilityAuthorization() remains nonisolated because it is fire and forget, and
  its work hops onto the actor internally
• the deinit is removed and init made private. A main-actor class has a non-isolated deinit
  that cannot touch isolated stored properties, and for a process-lifetime singleton it was
  unreachable regardless. Teardown is already explicit in applicationWillTerminate, which calls
  stopMonitoringAccessibilityAuthorization() directly

Net effect on the file is 43 insertions and 80 deletions.

Results

Project-wide diagnostics that the compiler marks as this is an error in the Swift 6 language mode, measured on clean Debug builds before and after:

	Before	After
Project total	227	164
XPCHelperClient.swift	56	0
components/OSD/Managers/XPC/BrightnessManager.swift	18	10
components/OSD/Managers/LunarManager.swift	13	14

BrightnessManager improves without being touched, because it calls into this client.

LunarManager gains one: sending value of non-Sendable type 'LunarEventListener'. Now that
startLunarEventStream(listener:) is main-actor isolated, passing a non-Sendable listener into it
from a detached task is a crossing. That is a pre-existing problem in LunarManager which this
change surfaces rather than causes, and that file is already on the list for its own pass.

One practical note for planning: per-file counts shift as these land, so the remaining split is
worth re-measuring before each PR rather than working from the original table in #1054.

Testing

Tested on a locally signed build, since the ad-hoc signed helper in a stock Debug build can't be
granted Accessibility. That signing change isn't part of this PR.

Accessibility prompt works, brightness, volume and keyboard backlight all show in the notch and
take effect, toggling Replace System OSD off and on restarts the monitor fine, and quitting from
the menu exits cleanly. Didn't test the Lunar paths since I don't have Lunar installed.

Relates to #1054. Further steps remain.

[theboringhumane]

fixed in #1496