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/sstv-general.js

@@ -802,7 +802,13 @@ const SSTVGeneral = (function() {
      * Delete a single image
      */
     async function deleteImage(filename) {
-        if (!confirm('Delete this image?')) return;
+        const confirmed = await AppFeedback.confirmAction({
+            title: 'Delete Image',
+            message: 'Delete this image? This cannot be undone.',
+            confirmLabel: 'Delete',
+            confirmClass: 'btn-danger'
+        });
+        if (!confirmed) return;
         try {
             const response = await fetch(`/sstv-general/images/${encodeURIComponent(filename)}`, { method: 'DELETE' });
             const data = await response.json();
@@ -822,7 +828,13 @@ const SSTVGeneral = (function() {
      * Delete all images
      */
     async function deleteAllImages() {
-        if (!confirm('Delete all decoded images?')) return;
+        const confirmed = await AppFeedback.confirmAction({
+            title: 'Delete All Images',
+            message: 'Delete all decoded images? This cannot be undone.',
+            confirmLabel: 'Delete All',
+            confirmClass: 'btn-danger'
+        });
+        if (!confirmed) return;
         try {
             const response = await fetch('/sstv-general/images', { method: 'DELETE' });
             const data = await response.json();