[good-first-issue] Add aria-label to theme toggle button

[Metrohan]

What to build / Ne yapılacak

EN: ThemeToggle.jsx renders an icon-only button with only a title attribute — no accessible name for screen readers, since title is not reliably announced by all assistive tech. Add a proper aria-label reflecting the action the button performs.

TR: ThemeToggle.jsx sadece ikon içeren bir buton render ediyor ve yalnızca title özniteliği var — bu, ekran okuyucular tarafından güvenilir şekilde duyurulmuyor. Butonun yaptığı eylemi yansıtan bir aria-label eklenmeli.

File

frontend/src/components/ThemeToggle.jsx

Acceptance criteria

• Add aria-label that reflects the current action, e.g. "Koyu moda geç" / "Açık moda geç" (mirroring the existing title text)
• Keep the existing title attribute for mouse users (tooltip)
• No visual/behavioral change
• Verify with a screen reader, axe, or Lighthouse accessibility audit that the button now has an accessible name

Blocked by

None — can start immediately.