Android: add optional proxy autostart after boot

[Regstar2]

Summary

Implements the code changes for #96 in an isolated branch.

• adds an opt-in boot autostart preference, disabled by default;
• registers a non-exported BOOT_COMPLETED receiver with RECEIVE_BOOT_COMPLETED;
• reuses ProxyService.ACTION_START and the existing foreground notification path;
• centralizes saved proxy startup configuration so the receiver and service use the same persisted source;
• skips disabled, invalid SOCKS5, and already-running cases;
• adds privacy-safe BOOT_* logging;
• adds App settings UI plus RU/EN strings;
• adds unit coverage and a reboot/device smoke checklist.

Validation

Automated CI should run on this PR. A physical/emulator reboot smoke remains required before merge because actual BOOT_COMPLETED delivery and foreground-service startup are platform/device behavior that unit tests cannot prove.

Refs #96

Issue #95 is intentionally untouched.