Load saved PoW filtering before the first geohash event

A restored geohash channel can receive messages before MainActivity's delayed initialization loads the saved proof-of-work preferences. During that window the handler sees the default disabled filter and accepts unmined messages. Initialize the existing preference manager when constructing the handler, before any event can be processed.

Fixes #421.

Validation: the new saved-filter regression fails against unchanged main and passes with this fix; the disabled-filter case and three existing signature tests also pass. The full phone JVM suite completed with 611 passing tests and three skipped tests. :app:lintDebug completed, reporting 305 errors and 347 warnings across the repository; no findings reference the changed files. This is not a clean-lint claim.

Draft pending the repository's physical Mesh Lab requirement for messaging changes. No physical-device or radio interoperability testing was performed. The change leaves packet formats, signatures, and the PoW threshold unchanged; its behavior change is applying the user's saved filter to the first received event.

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• Reviewed the diff and checked for an existing equivalent PR
• Linked the issue
• Added automated regression coverage