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.

src/stores/settings.js

@@ -23,6 +23,8 @@ export const useSettingsStore = defineStore('settings', () => {
     const backgroundType = ref('default') // 'default' | 'warm' | 'reading' | 'image'
     const backgroundImage = ref('')
     const backgroundOpacity = ref(0.2) // 0.05 - 0.50
+    // TTS Voice
+    const ttsInstruct = ref('')
 
     // --- Getters ---
     const uiLanguage = computed(() => {
@@ -70,6 +72,7 @@ export const useSettingsStore = defineStore('settings', () => {
                 }
                 if (data.backgroundImage) backgroundImage.value = data.backgroundImage
                 if (data.backgroundOpacity) backgroundOpacity.value = data.backgroundOpacity
+                if (typeof data.ttsInstruct === 'string') ttsInstruct.value = data.ttsInstruct
             }
         } catch {
             // Failed to load settings, use defaults
@@ -89,6 +92,7 @@ export const useSettingsStore = defineStore('settings', () => {
                 backgroundType: backgroundType.value,
                 backgroundImage: backgroundImage.value,
                 backgroundOpacity: backgroundOpacity.value,
+                ttsInstruct: ttsInstruct.value,
             }
             localStorage.setItem('llm-in-text-settings', JSON.stringify(data))
         } catch {
@@ -107,6 +111,7 @@ export const useSettingsStore = defineStore('settings', () => {
         backgroundType.value = 'default'
         backgroundImage.value = ''
         backgroundOpacity.value = 0.2
+        ttsInstruct.value = ''
         saveSettings()
     }
 
@@ -122,6 +127,7 @@ export const useSettingsStore = defineStore('settings', () => {
             backgroundType,
             backgroundImage,
             backgroundOpacity,
+            ttsInstruct,
         ],
         () => {
             saveSettings()
@@ -141,6 +147,7 @@ export const useSettingsStore = defineStore('settings', () => {
         backgroundType,
         backgroundImage,
         backgroundOpacity,
+        ttsInstruct,
         uiLanguage,
         t,
         initialMarkdown,