#10764 Fix blank Vault and sidebar icons through the shared renderer

Fixes #4476.

This is a clean replacement for PR #4480. The original branch had drifted 6,687 commits behind current main and carried unrelated CI-watchdog history; this branch starts at current main and contains only the two icon regression/fix commits.

Root cause

The remaining Vault/workspace/sidebar SF Symbol call sites still entered SwiftUI as lazy template providers. On macOS 15, especially Intel, the first materialization can occur before the effective appearance is resolved, producing a blank icon layer while text remains. Sidebar re-show and Light/Dark toggles accidentally retriggered that lifecycle.

Fix

• Route Vault section controls, agent fallbacks, folder sections, previews, and popovers through the existing AppKit-materialized CmuxSystemSymbolImage path.
• Route Task Manager agent/system icons through the same concrete bitmap path.
• Preserve the existing appearance-aware CmuxAppKitSupportUI renderer and package lifecycle ownership from current main.
• Add runtime renderer/lifecycle regression coverage, including visible non-template pixels and last-good-image preservation.
• Keep all changes localized; no submodule changes.

Verification

• Swift frontend parse of all touched Swift files.
• git diff --check.
• scripts/check-pbxproj.sh.
• scripts/lint-pbxproj-test-wiring.sh.
• No local Xcode build or tests run by request; cloud build/test remains the handoff gate.

The old PR #4480 should be superseded by this PR; no merge is requested here.

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Summary by CodeRabbit

• New Features

  • Added consistent icon styling across session, task, grouping, search, and transcript views.
  • Improved agent and folder icons with shared rendering and workspace-based fallbacks.
  • Added fallback support for system symbols when primary icons are unavailable.
  • Added accessibility labels for reload and clear-search controls.

• Bug Fixes

  • Prevented blank or transparent icons from appearing during rendering transitions.
  • Improved icon updates when fallback images change dynamically.

• Tests

  • Added coverage for fallback rendering, workspace icons, transparent images, and dynamic icon updates.