fix(ui): unify colored-control text — white on all filled, both themes

The text color on filled colored controls was inconsistent: contained
primary resolved to white in both schemes, but error/success/warning/info
used a per-scheme contrastText (#fff light, #1a1a1a dark). So in DARK mode
a primary (red) button showed white text while an error (red) button —
e.g. every ConfirmDialog delete confirm, the contained "Smazat", and the
dashboard quick-action tiles — showed near-black text. "Black text on one
red button, white on another."

Unify on a single rule: every FILLED colored surface gets WHITE text/glyph
in BOTH themes. The palette .main stays bright in dark mode (it drives text,
outlined buttons and row tints), but a bright fill makes white illegible, so
filled surfaces drop to a darker fill in dark mode (new FILLED_DARK_BG, ≈ the
light-scheme shades; all ≥4.5:1 with white):
- theme MuiButton: contained error/success/warning/info -> white text +
  darker dark-mode fill (disabled state preserved).
- theme MuiChip: filled error/success/warning/info -> white label + darker
  dark-mode fill (covers StatusChip + status pills).
- StatCard icon badges: white glyph + darker dark-mode tile.

Plus two consistency cleanups:
- Detail-page "Smazat" unified to variant=outlined color=error (Offer +
  Invoice now match Order/Project/Warehouse; the contained->confirm pattern
  stays: subtle trigger, strong contained confirm dialog).
- "Zrušit filtry" reset buttons -> color=inherit (neutral), matching the
  rest of the secondary-button family (were default outlined-primary).

Neutral (color=inherit) text/outlined buttons were already correct (label =
ambient text color) and are untouched. tsc -b --noEmit, npm run build and
vitest 152/152 all clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/pages/OfferDetail.tsx

@@ -1184,7 +1184,11 @@ export default function OfferDetail() {
               </Button>
             )}
             {isEdit && hasPermission("offers.delete") && (
-              <Button onClick={() => setDeleteConfirm(true)} color="error">
+              <Button
+                onClick={() => setDeleteConfirm(true)}
+                variant="outlined"
+                color="error"
+              >
                 Smazat
               </Button>
             )}