Refine mouse and header selection and preserve native search shortcuts

[jonaraphael]

Cells take their selected color on mouse-down and retain that paint through release. A first header click selects its whole column; a second separate click on the same header selects just the header cell. A true double-click edits the header, and dragging a selected header still reorders its column or group. Shift selection, right-click preservation, resize boundaries, serial-index selection and native cell editing retain their guards. Existing palette values remain unchanged.

Cmd/Ctrl+F and H open CSV Find/Replace only without Shift or Alt. Cmd/Ctrl+Shift+F/H remain available to native workspace search and replace. Held-arrow navigation follows the latest selected cell once per display frame; Shift navigation keeps the original anchor while the moving endpoint stays visible, including sticky/frozen/filter geometry. Cmd/Ctrl+Shift+Arrow extends from that endpoint and retains whole-row or whole-column selection shape.

This PR is stacked on #124 and includes the shared #111 clipboard follow-up: ordinary neighbors remain unquoted, the append row is never copied or cut, and a ghost-only gesture leaves clipboard/source untouched. Real empty source cells remain.

Validation:

• Parent branches are synchronized. The update preserves every production and regression-test file; only README clipboard wording was aligned.
• Node 20.20.0: fresh compile and 253 branch tests pass. Lint has 0 errors and 87 existing warnings; diff checks pass.
• Generated-provider/shipped-webview regressions cover separate header clicks, true double-click editing, column/group reorder, Shift/right-click, link/serial clicks and resize reset. Modified Find/Replace tests cover both platform modifiers with Shift/Alt combinations.
• Existing mouse-down, held-arrow, modified range and native editing tests remain green. Earlier Chromium probes verified identical mouse-down/up selection paint in four themes and no offscreen frames across held-arrow repeat/reversal cases.
• Current combined source passes 658 tests, including the routing follow-up. Chromium confirms first-click whole-column selection, second separate header-only selection, true double-click editing and no CSV Find popup on Cmd+Shift+F. Column/ghost copy behavior is verified with an actual generated page and mocked VS Code host.
• The maintainer’s native Copied table does not saved into clipboard #105 clipboard smoke test passes. Their prior isolated Cannot jump to search result in text editor when CSV extension is enabled #104 candidate failed. The updated Preserve native search destinations and CSV editor lifecycle #110 routing fix now passes native VS Code SearchView commands, with the exact match selected in a single text tab; the combined build passes that check too. This selection PR adds the separate global-shortcut guard.