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/ook.js

@@ -498,15 +498,27 @@ var OokMode = (function () {
         input.value = '';
     }
 
-    function removePreset(freq) {
-        if (!confirm('Remove preset ' + freq + ' MHz?')) return;
+    async function removePreset(freq) {
+        const confirmed = await AppFeedback.confirmAction({
+            title: 'Remove Preset',
+            message: 'Remove preset ' + freq + ' MHz?',
+            confirmLabel: 'Remove',
+            confirmClass: 'btn-danger'
+        });
+        if (!confirmed) return;
         var presets = loadPresets().filter(function (p) { return p !== freq; });
         savePresets(presets);
         renderPresets();
     }
 
-    function resetPresets() {
-        if (!confirm('Reset to default presets?')) return;
+    async function resetPresets() {
+        const confirmed = await AppFeedback.confirmAction({
+            title: 'Reset Presets',
+            message: 'Reset to default presets?',
+            confirmLabel: 'Reset',
+            confirmClass: 'btn-danger'
+        });
+        if (!confirmed) return;
         savePresets(DEFAULT_FREQ_PRESETS.slice());
         renderPresets();
     }