Polish PlanViewer toolbars, Query Store headers, and tab accent

[erikdarlingdata]

Summary

Small set of in-session polish items for Performance Studio:

• Query Store grid column headers — dropped (Local) from Last Executed (Time display dropdown already controls timezone), shortened Physical Reads → Phys Reads, and widened CPU / Duration / Memory / Phys Reads / Executions columns by 5–15px so (ms) and (MB) suffixes no longer truncate in the default layout.
• Toolbar density — reduced button padding 10,0 → 8,0, StackPanel Spacing 6 → 4, and pipe-separator margins 4,0 → 2,0 (and 12,0 → 6,0) across both QuerySessionControl and PlanViewerControl toolbars. Dropped redundant per-button margins now that Spacing handles them.
• Wait palette collisions — Worker Thread #19ff25 → #52E3B5 (teal, distinct from CPU green) and Parallelism #fd01d3 → #7B4FFF (vivid purple, out of the pink/magenta cluster) so neighboring wait categories on the stacked wait profile bar read as different colors.
• Tab active-state accent — added SystemAccentColor (and its Dark1/Dark2/Light1/Light2 variants) to DarkTheme.axaml. Fluent's TabItem selected pipe reads from SystemAccentColor, so Plan 1 / Plan 2 tabs now get the brand-blue underline consistently instead of the Fluent default teal.

Test plan

• Launch app, open Query Store, verify column headers fully readable without horizontal scroll
• Open an .sqlplan with multiple statements, hover toolbars, verify tighter spacing still readable and usable
• Load a plan with parallel + worker-thread waits and verify the stacked wait bar shows distinct colors for each category
• Open two plan tabs, switch between them, confirm the active tab shows a brand-blue underline

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