fix: correct UTF-8 encoding in base64 decode for file paths fix #20

The old decodeBase64 implementation tried atob() first which returns
garbled text for UTF-8 content (e.g., Chinese characters), and only
fell back to correct UTF-8 decoding when atob() threw an exception
(which it doesn't for valid base64).

Changes:
- Add encodeBase64/decodeBase64 utilities to web/src/lib/utils.ts
  using TextEncoder/TextDecoder for proper UTF-8 support
- Update file.tsx to use shared decodeBase64 instead of buggy local impl
- Update files.tsx to use shared encodeBase64 instead of deprecated
  escape/unescape approach

web/src/routes/sessions/file.tsx

@@ -9,18 +9,7 @@ import { useAppGoBack } from '@/hooks/useAppGoBack'
 import { useCopyToClipboard } from '@/hooks/useCopyToClipboard'
 import { queryKeys } from '@/lib/query-keys'
 import { langAlias, useShikiHighlighter } from '@/lib/shiki'
-
-function decodeBase64(value: string): { text: string; ok: boolean } {
-    try {
-        return { text: atob(value), ok: true }
-    } catch {
-        try {
-            return { text: decodeURIComponent(escape(atob(value))), ok: true }
-        } catch {
-            return { text: '', ok: false }
-        }
-    }
-}
+import { decodeBase64 } from '@/lib/utils'
 
 function decodePath(value: string): string {
     if (!value) return ''

web/src/routes/sessions/files.tsx

@@ -7,14 +7,7 @@ import { useAppGoBack } from '@/hooks/useAppGoBack'
 import { useGitStatusFiles } from '@/hooks/queries/useGitStatusFiles'
 import { useSession } from '@/hooks/queries/useSession'
 import { useSessionFileSearch } from '@/hooks/queries/useSessionFileSearch'
-
-function encodePath(value: string): string {
-    try {
-        return btoa(value)
-    } catch {
-        return btoa(unescape(encodeURIComponent(value)))
-    }
-}
+import { encodeBase64 } from '@/lib/utils'
 
 function BackIcon(props: { className?: string }) {
     return (
@@ -255,8 +248,8 @@ export default function FilesPage() {
 
     const handleOpenFile = useCallback((path: string, staged?: boolean) => {
         const search = staged === undefined
-            ? { path: encodePath(path) }
-            : { path: encodePath(path), staged }
+            ? { path: encodeBase64(path) }
+            : { path: encodeBase64(path), staged }
         navigate({
             to: '/sessions/$sessionId/file',
             params: { sessionId },