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/components/SettingsPanel.vue

@@ -202,10 +202,10 @@ const switchView = (view) => {
               v-model.number="store.backgroundOpacity" 
               class="range-slider"
             />
-          </div>
-        </section>
+</div>
+</section>
 
-        <!-- View Switch -->
+<!-- View Switch -->
         <section class="settings-section">
           <h3>{{ t('view') || '视图' }}</h3>
           <div class="view-switch">
@@ -306,12 +306,27 @@ const switchView = (view) => {
               <option value="AUD">AUD ($)</option>
               <option value="CAD">CAD ($)</option>
             </select>
-          </div>
-        </section>
+</div>
+</section>
 
-        <!-- About -->
-        <section class="settings-section">
-          <h3>{{ t('about') }}</h3>
+<!-- TTS Settings -->
+<section class="settings-section">
+<h3>{{ t('ttsSettings') || '语音设置' }}</h3>
+<div class="form-group">
+  <label>{{ t('voiceInstruction') || '声音描述' }}</label>
+  <input
+    type="text"
+    v-model="store.ttsInstruct"
+    class="select-input"
+    :placeholder="t('voiceInstructionPlaceholder') || '例如：用温柔的语气说'"
+  />
+  <p class="help-text">{{ t('voiceInstructionDesc') || '描述你想要的声音风格，如语气、情感等' }}</p>
+</div>
+</section>
+
+<!-- About -->
+<section class="settings-section">
+<h3>{{ t('about') }}</h3>
           <div class="about-card">
             <h4>llm-in-text</h4>
             <p>A smart Markdown editor with local LLM intelligence.</p>
@@ -641,4 +656,3 @@ const switchView = (view) => {
   font-weight: 600;
 }
 </style>
-