Redesign merged provider switcher for high provider counts and keyboard use

[houserj898-del]

Summary

The merged-menu provider switcher works with a few providers, but it gets dense and hard to navigate as more providers are enabled. It is also a custom NSView control, so keyboard and accessibility behavior need to be explicit.

Related: #203, #208, #266

Current behavior

• The merged menu uses a fixed card width.
• The provider switcher can wrap into multiple rows and shrink labels as provider count grows.
• Hover styling exists, but discoverability and keyboard focus behavior are limited.
• The token-account switcher uses the same custom-control pattern.

Why this matters

• The users most likely to enable Merge Icons are also the ones most likely to hit the scaling limits.
• Dense multi-row switchers are harder to scan quickly in a menu-bar UI.
• Custom controls in NSMenu need stronger keyboard and accessibility support than native menu items get for free.

Proposed direction

• Revisit the switcher layout for high provider counts: adaptive width, overflow/favorites, or a more compact representation.
• Add full keyboard navigation and visible focus treatment for provider and account switchers.
• Add explicit accessibility labels/tooltips for each segment.
• Revisit spacing/light-mode treatment as part of the same pass.

Acceptance criteria

• Merged mode remains readable with a larger provider set.
• Provider and token-account switchers are fully usable without a mouse.
• Controls expose clear accessibility labels and focus states.
• The redesign addresses or supersedes the UX problems reported in Max Provider View Issue #203, [Feedback] UI oddities #208, and Keyboard navigation of model #266.