feat: download dirs, file transfer resume, Wails desktop app, PWA, CI

Daemon:
- Per-network download directories (-download-dir flag, set_download_dir IPC)
- File transfer resume after disconnection: .tmp.meta sidecars survive
  interruption; resume_offset in file-accept lets sender seek and continue
- set_download_dir IPC command; download_dir reported in state_snapshot

Protocol:
- PeerMessage.ResumeOffset (EXT-006) for file transfer resume
- IpcMessage.ShareNetworks json tag changed from "networks" to "network_ids"
  to fix duplicate json tag collision with Networks []NetworkInfo

Desktop app (cmd/app):
- Wails v2 shell embedding daemon logic directly (no subprocess)
- System tray on Linux/Windows via getlantern/systray; macOS hides to Dock
- OS notifications for message_received and file_complete via Wails events
- notray build tag for headless/CI builds without GTK tray headers
- build-app.sh: builds web frontend, copies dist, runs wails build

Web / PWA:
- manifest.json + Apple touch icon meta tags for mobile "Add to Home Screen"
- PNG icons (192px, 512px, 180px) generated from SVG
- Wails EventsOn("notify") hook in App.tsx for native OS notifications

CI:
- .gitea/workflows/build.yml: server binaries cross-compiled for 5 platforms,
  desktop app for Linux amd64, release artifacts published on v* tags

Docs:
- README: download dir, file transfer resume, desktop app, PWA, CI sections
- EXTENSIONS.md: EXT-004 daemon mode marked shipped; EXT-006 resume added
- FUTURE.md: roadmap updated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

EXTENSIONS.md

@@ -117,8 +117,8 @@ proper signed invite (if the network enforces it) to be accepted by peers.
 ### New IPC commands
 
 ```jsonc
-{"type":"add_share","path":"/home/alice/Music"}                        // global
-{"type":"add_share","path":"/home/alice/Docs","networks":["abc123"]}   // scoped
+{"type":"add_share","path":"/home/alice/Music"}                                // global
+{"type":"add_share","path":"/home/alice/Docs","network_ids":["abc123"]}       // scoped
 {"type":"remove_share","path":"/home/alice/Music"}
 {"type":"list_shares"}
 ```
@@ -148,7 +148,7 @@ entries from all applicable share roots, with relative `path` fields
 
 ## EXT-004 — TURN Relay (browser mode)
 
-**Status:** implemented (browser mode); pending (daemon mode)  
+**Status:** implemented (browser mode + daemon mode)  
 **Affects:** ICE server configuration only, no wire changes
 
 The browser adapter reads `WASTE_CONFIG.turnURL` and `WASTE_CONFIG.turnSecret`
@@ -171,3 +171,56 @@ this field continue to use `name` for display and download requests.
 
 `MsgFileListReq` / `get` requests use `path` as the lookup key when present,
 falling back to `name` for backward compat with peers that don't send `path`.
+
+---
+
+## EXT-006 — File Transfer Resume
+
+**Status:** implemented (daemon mode)  
+**Affects:** `file-accept` wire message (additive field)
+
+### Motivation
+
+A transfer interrupted mid-stream (peer disconnect, network drop) can be
+continued from where it left off on the next offer of the same file, avoiding
+a full re-download.
+
+### Protocol change
+
+`file-accept` gains one optional field:
+
+```json
+{ "type": "file-accept", "xid": "...", "resume_offset": 65536 }
+```
+
+`resume_offset` is the number of bytes the receiver already has on disk.
+When non-zero, the sender seeks to that byte position before streaming.
+Peers that don't understand this field ignore it and send from the start —
+the receiver detects this by comparing incoming data to expected offset and
+will still verify the final SHA-256, but the partial bytes from the interrupted
+session will be overwritten (YAW/2-only interop degrades gracefully to a full
+re-download, not corruption).
+
+### Receiver behaviour
+
+1. On `file-offer`, the receiver scans its download directory for a `.tmp.meta`
+   sidecar whose `sha256` matches the offer.
+2. If found, the corresponding `.tmp` file's size is the resume offset. This is
+   sent back in `file-accept`.
+3. On DC open, the receiver opens the existing `.tmp` in append mode and
+   re-hashes its existing bytes to restore the SHA-256 state.
+4. On DC close with all bytes received, SHA-256 is verified. Success → sidecar
+   removed, file renamed to final path. Hash mismatch → both files removed.
+5. On DC close with fewer bytes than expected (interrupted again) → both files
+   kept for the next resume attempt.
+
+### Sidecar format
+
+Each in-progress `.tmp` file has a corresponding `.tmp.meta` JSON sidecar:
+
+```json
+{ "name": "archive.zip", "sha256": "abc...", "from": "<peer-id>", "size": 1048576 }
+```
+
+The sidecar is written when the transfer starts and removed on completion or
+corruption. Interrupted transfers keep the sidecar indefinitely.