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/main.cpp

@@ -112,6 +112,9 @@ static bool parse_json(const char* json, UsageData* out) {
     out->weekly_reset_mins = doc["wr"] | -1;
     strlcpy(out->status, doc["st"] | "unknown", sizeof(out->status));
     out->ok = doc["ok"] | false;
+    out->tokens_today = doc["tk"] | (long long)0;
+    out->cost_cents_today = doc["tc"] | 0;
+    out->messages_today = doc["tn"] | 0;
     out->valid = true;
     return true;
 }