Harden desktop action wiring and polish app shell UI for a production-quality control surface

[Copilot]

The desktop control surface looked unfinished and several controls appeared non-functional. This change tightens runtime integration for action buttons and upgrades the UI treatment so the app reads as intentional rather than placeholder.

• Runtime command wiring

  • Updated Tauri invocation resolution to support both global surfaces used by current runtime contexts:
    • window.__TAURI__?.core?.invoke
    • window.__TAURI_INTERNALS__?.invoke
  • Introduced a single runtime capability gate (hasDesktopRuntime) used to drive action availability.

• Non-desktop fallback behavior

  • Added explicit browser-preview mode handling:
    • Disables desktop-only controls (connect, settings save, split controls, diagnostics, profile export/import) when Tauri invoke is unavailable.
    • Updates UI state text to communicate preview mode instead of leaving controls to fail on interaction.

• UI professionalism pass

  • Refined topbar and card presentation (subtle blur/surface depth).
  • Improved primary-action emphasis.
  • Standardized disabled-state affordances across button variants so unavailable actions are visually unambiguous.

const tauriInvoke = window.__TAURI__?.core?.invoke || window.__TAURI_INTERNALS__?.invoke;
const hasDesktopRuntime = typeof tauriInvoke === 'function';

function setRuntimeUiState() {
  if (hasDesktopRuntime) return;
  [connectBtn, saveSettingsBtn, syncSplitBtn, stopSplitBtn, runDiagnosticsBtn, exportProfileBtn, importProfileFile]
    .forEach((el) => { if (el) el.disabled = true; });
  topbarState.textContent = 'Browser Preview';
}