Fix macOS Tahoe stoplight positioning and build on macOS 26

On macOS Tahoe (26+), the default title bar is 32px with 14px buttons,
so the old formula (button_height + PAD_Y = 14 + 18 = 32) produced no
change. Add TITLEBAR_EXTRA_HEIGHT to push the title bar taller than
the Tahoe default. Use OnceLock to capture the original default height
so repeated calls don't accumulate extra pixels.

Also update CI runners to macos-26 for Tahoe SDK builds and adjust
frontend padding for larger stoplights.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/release-app.yml

@@ -13,12 +13,12 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - platform: "macos-latest" # for Arm-based Macs (M1 and above).
+          - platform: "macos-26" # for Arm-based Macs (M1 and above).
             args: "--target aarch64-apple-darwin"
             yaak_arch: "arm64"
             os: "macos"
             targets: "aarch64-apple-darwin"
-          - platform: "macos-latest" # for Intel-based Macs.
+          - platform: "macos-26-intel" # for Intel-based Macs.
             args: "--target x86_64-apple-darwin"
             yaak_arch: "x64"
             os: "macos"