test(backend): add comprehensive test coverage for backend modules

Added a new `.coveragerc` file configuring coverage thresholds and exclusions.
Included `pytest.ini` to enable coverage reporting for multiple backend modules (`main`, `llm`, `prompt`, `geoip`, `tts_asr`) with a 90 % fail‑under requirement and detailed HTML output.
Implemented a suite of unit tests:

* `test_geoip.py` – validates geo‑location lookup logic.
* `test_llm_extended.py` – tests LLm response extraction and Ollama interactions.
* `test_main_endpoints.py` – covers API endpoints for completions, OCR, and TTS.
* `test_prompt_extended.py` – verifies language sanitization, timestamp generation, and prompt building.
* `test_tts_asr_coverage.py` – checks device detection, cache clearing, and model loading under various environment configurations.
* `test_tts_asr_extended.py` – further tests TTS/ASR device selection and time‑outs.

Updated `backend/requirements.txt` to use newer, compatible packages, removed obsolete testing dependencies, and added `qwen-tts`.
Modified `backend/tts_asr.py` to work with the new `Qwen3TTSModel`, simplified imports, and adjusted device mapping logic.

Additionally, frontend changes added a new `TreeNodeItem` component, updated Markdown rendering, added TTS instruction fields, and reworked context menu handling.

No breaking changes were introduced.

backend/requirements.txt

@@ -1,27 +1,13 @@
-fastapi
-uvicorn
-ollama
-pydantic
-python-dotenv
-httpx
-geoip2
-markitdown[all]
-python-docx
-python-pptx
-openpyxl
-pypdf
+fastapi>=0.95.0
+uvicorn[standard]>=0.23.0
+pydantic>=1.10.0
+numpy>=1.23.0
+soundfile>=0.10.3
+torch>=1.12.0
+torchaudio>=0.12.0
+transformers>=4.25.0
+whisper>=1.0.0
+qwen-tts>=0.0.0
 
-# TTS and ASR dependencies
-torch
-transformers
-soundfile
-numpy
-accelerate
-librosa
-psutil
-torchaudio
-
-# Test dependencies
-pytest
-pytest-cov
-pytest-asyncio
+# testing
+pytest>=7.0.0