feat(ui): use StackOne brand mark instead of bell icons

[StuBehan]

What

Replace the generic bell iconography with the official StackOne brand mark across the app's chrome.

Surface	Before	After
Menu bar status item	bell (SF Symbol)	StackOne mark
Panel top tab strip	bell.badge.fill	StackOne mark (14pt)
Shared PageFooter	bell.badge.fill (left)	removed — footer is hint-pills only

The "No nudges yet" empty state keeps bell.slash — that one is semantic (no nudges), not decorative chrome.

How

The mark is reproduced from the official 16×16 icon SVG (stackone-logos.com) as a resolution-independent NSBezierPath vector, rather than a bundled raster:

• Exact to the brand geometry (SVG path data translated directly), and point-symmetric by construction.
• Crisp at any menu-bar height and on Retina — a raster .ico/PNG would be pixel-locked and blur at off sizes.
• No binary asset to ship and no build.sh change.
• Authoritative brand green #00AF66 (the SVG's colour; the repo PNG was a slightly different #00B979).

All three surfaces render from the single MenuBarController.brandMarkImage(height:), so there's one source of truth. It's a deliberately coloured (non-template) icon; the negative space is transparent and shows the menu bar through, which reads on both light and dark bars.

Testing

• swiftc -typecheck panel/*.swift shared/*.swift — clean.
• Verified live in the menu bar; previewed the mark on light/dark backgrounds and at ~15pt.

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