v2 Phase 4: Session screen — today's tokens and equivalent cost

Daemon: compute_today_usage() sums today's Claude Code token usage across the
local project transcripts (~/.claude/projects/**/*.jsonl), reading only files
modified today so the scan stays cheap, and prices it with Anthropic list rates
to show the equivalent API cost (the ccusage-style flex for subscription users).
The compact fields tk (total tokens), tc (cost in cents) and tn (message count)
piggyback onto the existing 60s BLE payload.

Firmware: UsageData gains tokens_today (64-bit) / cost_cents_today /
messages_today; parse_json reads tk/tc/tn; a new Session screen shows the cost
as the hero with total tokens and message count below, refreshed from ui_update
so it's current whenever opened. Wire SCREEN_SESSION to the real screen (was a
stub). Verified end to end: daemon sends e.g. tk=105374109 tc=26838 tn=663.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

firmware/src/data.h

@@ -9,4 +9,10 @@ struct UsageData {
     char status[16];         // "allowed" or "limited"
     bool ok;                 // data parse succeeded
     bool valid;              // false until first successful parse
+
+    // Today's Claude Code usage (Phase 4 — computed by the daemon from local
+    // transcripts). Tokens can exceed 2^31 on a heavy day, so use 64-bit.
+    long long tokens_today;     // total tokens today (input+output+cache)
+    int       cost_cents_today; // equivalent API cost today, US cents
+    int       messages_today;   // assistant messages today
 };