fix: honour a stated rate whatever the calendar says

Pay-now on a period due next week offered "Rate on the payday" and then
found a rate anyway, with only a passive hint to explain why. Two separate
problems behind that.

The real one: `resolve_price` tested `payday >= today` before it tested the
mode, so `manual` was silently ignored for any period not already
back-dated. "Pay at the rate we agreed" quietly did not, and a contract
pegged to a fixed rate honoured it only on late periods. A stated rate is
an instruction rather than a lookup, so it now wins outright and applies to
every period. `payday` and `current` keep their old order — for a payday
today or ahead there is no history to consult, so `payday` collapses into
`current`, which is the only defensible answer for a date that has not
happened.

The cosmetic one: the dialog's hint said "ignored unless the payday is
already in the past", sitting beside a control that plainly appeared to do
something — and was about to become wrong anyway, since manual now always
applies. It is replaced by a line that names the actual period: either
"Payday 2026-09-05 has not passed, so there is no historical rate to look
up — this will be priced at the current rate", or "Will convert at what BTC
was worth on <date>". The dialog header now shows which payday is being
settled, which was not visible at all before.

The contract dialogs lose the same stale hint; their label becomes
"Pricing" rather than "Price a late payday at", which no longer describes
manual mode.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018jy52j9GRZ6XKa1Zt21LLj

tests/test_pricing.py

@@ -168,3 +168,37 @@ def test_memo_of_a_sat_contract_has_no_rate():
 
 def test_memo_without_a_rate_still_states_the_fiat_amount():
     assert services.payout_memo(eur(label="x"), PAST) == "x — 2026-05-15 · 800 EUR"
+
+
+# --- an explicit rate is an instruction, not a lookup ----------------------
+
+
+def test_manual_applies_to_a_payday_today(historical):
+    """ "Pay at the rate we agreed" is as meaningful for a payday now as for
+    one last month — the calendar must not quietly override it."""
+    p = price(eur(pricing_mode=PricingMode.manual, manual_rate=100_000), payday=TODAY)
+    assert (p.source, p.rate) == ("manual", 100_000)
+    assert p.amount == 800_000
+
+
+def test_manual_applies_to_a_future_payday(historical):
+    p = price(
+        eur(pricing_mode=PricingMode.manual, manual_rate=100_000),
+        payday=date(2026, 12, 1),
+    )
+    assert p.source == "manual"
+
+
+def test_manual_override_beats_a_future_payday(historical):
+    """Paying next week's period early, at a stated rate."""
+    p = price(
+        eur(), payday=date(2026, 12, 1), mode=PricingMode.manual, manual_rate=50_000
+    )
+    assert (p.source, p.rate) == ("manual", 50_000)
+
+
+def test_payday_mode_on_a_future_payday_falls_to_the_live_rate(historical):
+    """There is no historical rate for a date that has not happened."""
+    p = price(eur(pricing_mode=PricingMode.payday), payday=date(2026, 12, 1))
+    assert p.source == "current"
+    assert historical == []