fix: keep the precise locator on digital QSOs; make the manual lookup real

Grid on the UDP path — WSJT-X and MSHV can only send a FOUR-character grid,
that is all the FT8 protocol carries. The enrichment rule there is "fill only
what is empty", so the coarse JN05 always won and QRZ's JN05JG was thrown away:
roughly 100 km of accuracy discarded on every digital QSO. The lookup grid is
now taken when it EXTENDS the received square. A lookup that disagrees outright
(JN18 against JN05) is not a refinement — the station may be portable and what
came over the air is then the better record. Both rules are pinned by tests.

Manual fetch in the QSO editor — it read the CACHE, valid for 30 days. An
operator who upgraded their QRZ subscription went on getting the thin
free-account record and clearing the cached row by hand was the only way out.
The button now forces a lookup that bypasses the cache, reaches the provider and
REFRESHES the stored row, with a 15 s budget instead of 2 s: a deliberate click
must reach the network, where giving up early would fall back to cty.dat and
look like nothing happened. The automatic type-ahead lookup keeps the cache,
which is where it earns its keep.

Same fetch: it used `??`, which only guards against null. Go marshals an unset
string as "", so a QRZ record with no grid BLANKED the grid already in the QSO.
The lookup still wins — that is the point of asking for it — but an empty
result no longer erases a good value.

frontend/src/lib/theme.tsx

@@ -14,7 +14,11 @@ export const CONCRETE_THEMES: Exclude<ThemeChoice, 'auto'>[] = [
 ];
 
 export const LS_KEY = 'opslog.theme';
-const DEFAULT: ThemeChoice = 'light-warm';
+// A fresh install starts DARK. A shack is usually a dim room and the screen is
+// looked at for hours; every other logger defaults the same way. Graphite
+// specifically, because that is what 'auto' already resolves to for a dark
+// system — so the two paths agree instead of landing on different darks.
+const DEFAULT: ThemeChoice = 'dark-graphite';
 const ALL: ThemeChoice[] = ['auto', ...CONCRETE_THEMES];
 
 function systemDark(): boolean {