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/meshtastic.html

@@ -132,7 +132,7 @@
     <div class="signal-details-modal-content">
         <div class="signal-details-modal-header">
             <h3>Configure Channel <span id="meshModalChannelIndex">0</span></h3>
-            <button class="signal-details-modal-close" onclick="Meshtastic.closeChannelModal()">&times;</button>
+            <button class="signal-details-modal-close" onclick="Meshtastic.closeChannelModal()" aria-label="Close channel config">&times;</button>
         </div>
         <div class="signal-details-modal-body">
             <div class="signal-details-section">
@@ -175,7 +175,7 @@
     <div class="signal-details-modal-content">
         <div class="signal-details-modal-header">
             <h3>Traceroute to <span id="meshTracerouteDest">--</span></h3>
-            <button class="signal-details-modal-close" onclick="Meshtastic.closeTracerouteModal()">&times;</button>
+            <button class="signal-details-modal-close" onclick="Meshtastic.closeTracerouteModal()" aria-label="Close traceroute">&times;</button>
         </div>
         <div class="signal-details-modal-body">
             <div id="meshTracerouteContent" class="mesh-traceroute-content">