Fix OOM crash on file send: lazy chunk reads, single transmitter session, correct modem profile

[Copilot]

Sending any non-trivial file caused an immediate out-of-memory crash, followed by transmission timeouts on every frame even if memory was sufficient.

Root causes & fixes

1. OOM — full file loaded into memory before transmission (sender-sm.ts)

file.arrayBuffer() was called on the entire file upfront. Replaced with per-frame lazy reads:

// Before: entire file in RAM
const fileBuffer = await this.file.arrayBuffer();

// After: one 4 KB slice at a time
const chunkBuffer = await this.file.slice(start, end).arrayBuffer();

Peak memory is now O(1 frame) regardless of file size.

2. Timeout — wrong modem profile made every frame exceed the deadline (quiet-modem.ts)

audible-fsk-robust runs at 250 samples/symbol (~50 bytes/sec), so a single 4096-byte application frame takes ~81 s of audio — well over the 30 s SEND_TIMEOUT_MS. Switched to audible-fsk (50 samples/symbol, ~250 bytes/sec, ~16 s/frame) and raised the timeout to 60 s for a 3.7× margin.

3. Transmitter churn — new ScriptProcessorNode created per frame (quiet-modem.ts, sender-sm.ts)

Added TransmitterSession: one Quiet.transmitter is created for the whole transfer and reused across all frames via a mutable onFinishRef callback. The node is destroyed once after the final frame.

Supporting changes (framing.ts)

• PAYLOAD_SIZE exported so sender-sm.ts can compute frame counts from file.size without reading file contents.
• createFileDataFrameFromPayload() added — accepts an already-sliced buffer instead of requiring the full file.