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>

static/js/modes/wifi.js

@@ -59,12 +59,12 @@ const WiFiMode = (function() {
     /**
      * Check for agent mode conflicts before starting WiFi scan.
      */
-    function checkAgentConflicts() {
+    async function checkAgentConflicts() {
         if (typeof currentAgent === 'undefined' || currentAgent === 'local') {
             return true;
         }
         if (typeof checkAgentModeConflict === 'function') {
-            return checkAgentModeConflict('wifi');
+            return await checkAgentModeConflict('wifi');
         }
         return true;
     }
@@ -411,7 +411,7 @@ const WiFiMode = (function() {
         if (isScanning) return;
 
         // Check for agent mode conflicts
-        if (!checkAgentConflicts()) {
+        if (!await checkAgentConflicts()) {
             return;
         }
 
@@ -503,7 +503,7 @@ const WiFiMode = (function() {
         if (isScanning) return;
 
         // Check for agent mode conflicts
-        if (!checkAgentConflicts()) {
+        if (!await checkAgentConflicts()) {
             return;
         }