fix: resolve hydration mismatches in ThemeToggle and FileUpload components

[compiler041]

Summary

Fixes hydration mismatch errors caused by theme-dependent rendering and
browser extension attribute injection.

Closes #[paste issue number here]

Changes

src/components/ThemeToggle.tsx

• Added mounted state initialized to false
• Added useEffect to set mounted = true after hydration
• Renders a neutral disabled placeholder button during SSR to ensure
  server and client HTML match exactly
• Placeholder icon uses opacity-0 to prevent layout shift

src/components/FileUpload.tsx

• Added suppressHydrationWarning to the DropZone inner div
• Prevents false positive hydration errors from browser extensions
  injecting fdprocessedid attributes

Root Cause

useTheme() returns theme state that depends on localStorage, which
is unavailable during SSR. This causes the server to render with a
default theme while the client renders with the stored preference.

Testing

• No hydration errors in console on fresh load
• Theme toggle works correctly after hydration
• File upload drop zone functions normally
• Tested with and without form-autofill extensions

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