Web: Add operator properties panel (#176)

[erikdarlingdata]

Summary

• Click any plan tree node to open a right-side properties panel with full operator details
• Full parity with the desktop app's properties panel (37 sections, 200+ properties)
• Includes per-thread stats for parallel operators, all predicates, costs, memory, and root-only statement/compilation info
• Panel slides in from right with close button; clicking same node toggles it closed

Test plan

• Paste estimated plan → click nodes → verify General, Object, Costs, Rows, Predicates, Output sections
• Paste actual plan → verify Actual Statistics, Timing, I/O sections with per-thread breakdowns
• Click root node → verify Statement Info, Set Options, Hardware, Memory Grant Info sections
• Switch statements in multi-statement plan → panel clears
• All 64 tests pass, solution builds clean

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