reorganize docs into 04-clients/ and 05-features/

docs/04-clients/mobile.md

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+# Mobile Client
+
+> Status: Phase 3 — not yet started.
+> This document tracks intentions and open questions.
+
+---
+
+## Goal
+
+A native mobile client for iOS and Android.
+Not a PWA. Not a wrapper around the desktop client.
+
+## Open questions
+
+### UI framework
+
+egui on mobile is not practical today — touch input support is limited,
+and the immediate mode model doesn't map well to mobile interaction patterns.
+
+Candidates under consideration:
+
+| Option | Notes |
+|--------|-------|
+| Rust + custom egui mobile backend | Most consistent with desktop codebase, significant work |
+| Rust + Makepad | Rust-native UI designed for mobile, less mature |
+| Rust core + Flutter UI | Dart for UI, Rust for audio/networking via FFI |
+| Rust core + Swift/Kotlin UI | Platform-native UI, Rust for the important parts |
+
+Decision: deferred to Phase 3.
+
+### Audio
+
+Mobile audio APIs are significantly more constrained than desktop:
+
+- iOS: AVAudioSession, strict background audio rules
+- Android: AAudio / OpenSL ES, varying latency by device
+
+opus encoding is the same. cpal has partial mobile support.
+The audio pipeline will need platform-specific tuning.
+
+### Background operation
+
+Voice calls in the background require OS-level permission and
+platform-specific handling (CallKit on iOS, ConnectionService on Android).
+This is non-trivial and will be a significant portion of mobile dev effort.
+
+---
+
+## What mobile must support (MVP)
+
+- connect to a VNOX node
+- join voice channels
+- push-to-talk
+- text chat
+- identity (same keypair as desktop, importable via QR or keyfile)
+
+## What mobile explicitly will not do
+
+- host a node (gateway or voice-node)
+- run plugins
+- game overlay
+
+---
+
+## Timeline
+
+Mobile client is Phase 3, after:
+- Phase 1: desktop client + server MVP
+- Phase 2: overlay, permissions, friend system
+
+Estimated start: after Phase 2 is stable.