fix: cross-driver correctness, a11y, and faster command-palette, ux and ui improvement, rendering fps improvements

[broisnischal]

UI polish and cross-engine hardening across the studio shell, plus two
crash/UX fixes surfaced in testing.

Fixes

• AI chat stuck on loading spinner: the tabError boundary snippet was
  declared in a sibling block to the always-mounted AiChat that referenced
  it, so it was out of scope at runtime (ReferenceError). Moved it to the
  shared parent scope so every tab boundary can see it.
• StructureView emitted PostgreSQL-only DDL (ALTER COLUMN … TYPE … USING,
  COMMENT ON COLUMN) on every engine, running invalid SQL on MySQL/SQLite.
  Gated all structure edits behind an editStructure capability: editing
  is read-only with a clear "use the SQL console" notice off Postgres, and
  every staging path + runDdlDirect is guarded so invalid DDL can't run.
• Column dropdown jumped back to the top on every toggle (cmdk re-selects
  the first item and scrolls it into view). Preserve the list scroll
  position across non-closing selects in SearchableMenu.

Features / polish

• Command palette table search: removed the debounce (the scorer is
  sub-millisecond) and added separator-insensitive fuzzy matching — "-",
  "_", ".", and spaces are interchangeable, with prefix/word/acronym/
  substring/subsequence ranking. Instant and intuitive.
• DataTable: "End of results — N rows" marker in infinite-scroll mode;
  aria-live focused-cell announcements + grid roles for screen readers;
  early-break column culling for very wide tables (row hot path untouched).
• Restore focus to the triggering element when dialogs close (focus
  tracker + shared dialog content), fixing keyboard focus dropping to body.
• Raise per-table column cache (60 → 400) to remove switch-back flicker.

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