feat(i18n): add Turkish (tr) language support (#678)

* feat(i18n): add Turkish (tr) language support

Add comprehensive Turkish localization for the UI, following the
existing i18n pattern established by Japanese (#384), Russian (#514),
and German (#525) language support.

Changes:
- Add Turkish translation files for all 7 namespaces
  (auth, chat, codeEditor, common, settings, sidebar, tasks)
- Register Turkish locale in config.js with all resources
- Add Turkish entry to languages.js (value: tr, nativeName: Türkçe)
- Update .gitignore to allow src/i18n/locales/tr/tasks.json
  (matches existing en/ja/ru/de exceptions)

Translation details:
- 934 total strings translated (100% coverage, matches en.json key count)
- Translated by a native Turkish speaker with software engineering
  background; terminology reviewed against conventional Turkish
  tech community usage.
- Technical terms kept in English per Turkish dev community norms:
  Claude, Cursor, Codex, Gemini, CLI, MCP, PRD, JSON, YAML, stdio,
  http, commit, branch, token, prompt, minimap, sandbox, YOLO.
- Informal second-person singular (\"sen\") used throughout — fits the
  developer-facing nature of the UI.
- All interpolation placeholders preserved exactly (e.g. {{count}},
  {{projectName}}, {{email}}).
- i18next plural keys (_one/_other) kept intact.

Verification:
- Key structure parity with en.json confirmed (jq paths diff empty)
- All 38 unique interpolation variables preserved
- npm run build passes cleanly

* docs(readme): add Turkish README and language switcher links

Add README.tr.md — full Turkish translation of the main README,
following the structure of existing README.de.md / README.ja.md /
README.ko.md / README.ru.md / README.zh-CN.md.

Update the language switcher row in all 6 existing README variants
to include a Turkish link (matches the pattern used by #534 for the
German language link addition).

---------

Co-authored-by: Simos Mikelatos <simosmik@gmail.com>

.gitignore

@@ -137,6 +137,7 @@ tasks/
 !src/i18n/locales/ja/tasks.json
 !src/i18n/locales/ru/tasks.json
 !src/i18n/locales/de/tasks.json
+!src/i18n/locales/tr/tasks.json
 !src/i18n/locales/it/tasks.json
 
 # Git worktrees