feat: PoW overhaul — background mining, persistent settings, reaction PoW

[barrydeen]

Summary

• Background mining: Note PoW now runs in a PowManager owned by FeedViewModel, so the compose screen navigates back immediately and mining progress shows in the bottom status pill with a cancel button
• Persistent settings: PoW enable/difficulty for notes and reactions stored in SharedPreferences via PowPreferences, with a new settings screen accessible from the drawer
• Reaction PoW: SocialActionManager.toggleReaction() now mines inline PoW (default 12 bits, sub-second) before signing reactions when enabled

New files

• repo/PowPreferences.kt — SharedPreferences-backed PoW settings (note/reaction enable + difficulty, clamped 8-32)
• viewmodel/PowManager.kt — Background mining manager with PowStatus sealed class (Idle/Mining/Done/Failed)
• ui/screen/PowSettingsScreen.kt — Settings screen with enable switches and difficulty steppers

Modified files

• FeedViewModel — owns PowPreferences and PowManager, passes powPrefs to SocialActionManager
• ComposeViewModel — removes old blocking PoW mining, hands off to PowManager.submitNote() for background mining
• SocialActionManager — adds inline reaction PoW via Nip13.mine() when enabled
• ComposeScreen — removes difficulty adjuster and mining progress UI, shows "mined in background" banner
• FeedScreen/BroadcastStatusBar — extended to show PowStatus (mining progress + cancel, done, failed states)
• Navigation.kt — adds POW_SETTINGS route, passes powManager/powPrefs to compose screen, wires status pill
• WispDrawerContent — adds "Proof of Work" entry under Settings

Test plan

• Compose a note with PoW on — verify immediate navigation back, mining pill appears, note publishes after mining
• Start a high-difficulty note, tap cancel on pill — verify mining stops, no note published
• React to a post with reaction PoW enabled — verify the reaction event has a nonce tag
• Toggle PoW off in settings, close/reopen app — verify it's still off
• Start mining a note, try to compose another — verify Post button is disabled
• Compose with PoW disabled — verify immediate publish (no mining phase)