feat: add admin panel, Replicate AI translation, and document translation

- Admin panel (/admin) with JWT auth: configure Replicate API token,
  JigsawStack API key, model version, enable/disable AI translation,
  change admin password. Settings persisted in data/settings.json.

- Replicate AI translation: POST /api/translate/replicate uses
  JigsawStack text-translate model via Replicate API. Main page
  switches to client-side AI translation when enabled.

- Document translation tab: supports PDF, DOCX, XLSX, XLS, CSV.
  Excel/Word formatting fully preserved (SheetJS + JSZip XML manipulation).
  PDF uses pdf-parse extraction + pdf-lib reconstruction.
  Column selector UI for tabular data (per-sheet, All/None toggles).

- Updated README with full implementation documentation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/ColumnSelector.tsx

@@ -0,0 +1,107 @@
+import { FC, useState, useEffect } from "react";
+import {
+    Box, Checkbox, CheckboxGroup, VStack, HStack, Text, Button,
+    Accordion, AccordionItem, AccordionButton, AccordionPanel, AccordionIcon,
+    Badge
+} from "@chakra-ui/react";
+
+export type SheetColumnInfo = {
+    sheetName: string;
+    columns: string[];
+};
+
+export type ColumnSelection = {
+    sheetName: string;
+    columnIndices: number[];
+};
+
+type Props = {
+    sheetColumns: SheetColumnInfo[];
+    onChange: (selections: ColumnSelection[]) => void;
+};
+
+const ColumnSelector: FC<Props> = ({ sheetColumns, onChange }) => {
+    const [selections, setSelections] = useState<Record<string, Set<number>>>(() => {
+        const init: Record<string, Set<number>> = {};
+        sheetColumns.forEach(s => {
+            init[s.sheetName] = new Set(s.columns.map((_, i) => i));
+        });
+        return init;
+    });
+
+    useEffect(() => {
+        const result: ColumnSelection[] = sheetColumns.map(s => ({
+            sheetName: s.sheetName,
+            columnIndices: Array.from(selections[s.sheetName] ?? [])
+        }));
+        onChange(result);
+    }, [selections, sheetColumns, onChange]);
+
+    const toggleColumn = (sheetName: string, colIdx: number) => {
+        setSelections(prev => {
+            const set = new Set(prev[sheetName] ?? []);
+            if (set.has(colIdx)) set.delete(colIdx);
+            else set.add(colIdx);
+            return { ...prev, [sheetName]: set };
+        });
+    };
+
+    const selectAll = (sheetName: string, cols: string[]) => {
+        setSelections(prev => ({
+            ...prev,
+            [sheetName]: new Set(cols.map((_, i) => i))
+        }));
+    };
+
+    const selectNone = (sheetName: string) => {
+        setSelections(prev => ({ ...prev, [sheetName]: new Set() }));
+    };
+
+    return (
+        <Box w="full" borderWidth={1} borderRadius="md" p={3}>
+            <Text fontWeight="bold" mb={2} fontSize="sm">Select Columns to Translate</Text>
+            <Accordion allowMultiple defaultIndex={sheetColumns.map((_, i) => i)}>
+                {sheetColumns.map(sheet => (
+                    <AccordionItem key={sheet.sheetName}>
+                        <AccordionButton>
+                            <Box flex="1" textAlign="left" fontSize="sm" fontWeight="semibold">
+                                {sheet.sheetName}
+                                <Badge ml={2} colorScheme="lingva">
+                                    {selections[sheet.sheetName]?.size ?? 0}/{sheet.columns.length}
+                                </Badge>
+                            </Box>
+                            <AccordionIcon />
+                        </AccordionButton>
+                        <AccordionPanel pb={3}>
+                            <HStack mb={2} spacing={2}>
+                                <Button size="xs" variant="outline" colorScheme="lingva"
+                                    onClick={() => selectAll(sheet.sheetName, sheet.columns)}>
+                                    All
+                                </Button>
+                                <Button size="xs" variant="outline"
+                                    onClick={() => selectNone(sheet.sheetName)}>
+                                    None
+                                </Button>
+                            </HStack>
+                            <VStack align="start" spacing={1} maxH="150px" overflowY="auto">
+                                {sheet.columns.map((col, idx) => (
+                                    <Checkbox
+                                        key={idx}
+                                        size="sm"
+                                        isChecked={selections[sheet.sheetName]?.has(idx) ?? false}
+                                        onChange={() => toggleColumn(sheet.sheetName, idx)}
+                                        colorScheme="lingva"
+                                    >
+                                        <Text fontSize="xs">{col}</Text>
+                                    </Checkbox>
+                                ))}
+                            </VStack>
+                        </AccordionPanel>
+                    </AccordionItem>
+                ))}
+            </Accordion>
+        </Box>
+    );
+};
+
+export default ColumnSelector;