feat: UI/UX overhaul — CSS cleanup, accessibility, error handling, inline style extraction

Phase 0 — CSS-only fixes:
- Fix --font-mono to use real monospace stack (JetBrains Mono, Fira Code, etc.)
- Replace hardcoded hex colors with CSS variables across 16+ files
- Merge global-nav.css (507 lines) into layout.css, delete original
- Reduce !important in responsive.css from 71 to 8 via .app-shell specificity
- Standardize breakpoints to 480/768/1024/1280px

Phase 1 — Loading states & SSE connection feedback:
- Add centralized SSEManager (sse-manager.js) with exponential backoff
- Add SSE status indicator dot in nav bar
- Add withLoadingButton() + .btn-loading CSS spinner
- Add mode section crossfade transitions

Phase 2 — Accessibility:
- Add aria-labels to icon-only buttons across mode partials
- Add for/id associations to 42 form labels in 5 mode partials
- Add aria-live on toast stack, enableListKeyNav() utility

Phase 3 — Destructive action guards & list overflow:
- Add confirmAction() styled modal, replace all 25 native confirm() calls
- Add toast cap at 5 simultaneous toasts
- Add list overflow indicator CSS

Phase 4 — Inline style extraction:
- Refactor switchMode() in app.js and index.html to use classList.toggle()
- Add CSS toggle rules for all switchMode-controlled elements
- Remove inline style="display:none" from 7+ HTML elements
- Add utility classes (.hidden, .d-flex, .d-grid, etc.)

Phase 5 — Mobile UX polish:
- pre/code overflow handling already in place
- Touch target sizing via --touch-min variable

Phase 6 — Error handling consistency:
- Add reportActionableError() to user-facing catch blocks in 5 mode JS files
- 28 error toast additions alongside existing console.error calls

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/partials/modes/bluetooth.html

@@ -14,13 +14,13 @@
                         </div>
 
                         <div class="form-group">
-                            <label>Adapter</label>
+                            <label for="btAdapterSelect">Adapter</label>
                             <select id="btAdapterSelect">
                                 <option value="">Detecting adapters...</option>
                             </select>
                         </div>
                         <div class="form-group">
-                            <label>Scan Mode</label>
+                            <label for="btScanMode">Scan Mode</label>
                             <select id="btScanMode">
                                 <option value="auto">Auto (Recommended)</option>
                                 <option value="bleak">Bleak Library</option>
@@ -30,7 +30,7 @@
                             </select>
                         </div>
                         <div class="form-group">
-                            <label>Transport</label>
+                            <label for="btTransport">Transport</label>
                             <select id="btTransport">
                                 <option value="auto">Auto (BLE + Classic)</option>
                                 <option value="le">BLE Only</option>
@@ -38,11 +38,11 @@
                             </select>
                         </div>
                         <div class="form-group">
-                            <label>Duration (seconds, 0 = continuous)</label>
+                            <label for="btScanDuration">Duration (seconds, 0 = continuous)</label>
                             <input type="number" id="btScanDuration" value="0" min="0" max="300" placeholder="0">
                         </div>
                         <div class="form-group">
-                            <label>Min RSSI Filter (dBm)</label>
+                            <label for="btMinRssi">Min RSSI Filter (dBm)</label>
                             <input type="number" id="btMinRssi" value="-100" min="-100" max="-20" placeholder="-100">
                         </div>
                         <button class="preset-btn" onclick="btCheckCapabilities()" style="width: 100%;">