Add inline error decorations for previews and editor

[zhemingfan]

Summary

The LSP already validates omics files, but diagnostics were only visible in the Problems panel and as editor squiggles — the previews, the strongest surface of the extension, were diagnostic-blind. This surfaces existing diagnostics where users actually look, without re-implementing what VS Code already renders (squiggles, overview ruler, minimap, editor F8).

What's new

• Preview row markers — a leading status column shows a severity glyph (● error / ▲ warning / ■ info), a subtle row tint, and a tooltip with [CODE] message for every row whose source line has a diagnostic. Clicking a marker reveals that line in the text editor. Wired into the shared VirtualTable and the VCF preview, so 13 previews get it from one code path; chart/tree/sequence previews degrade gracefully to no markers.
• Editor gutter icons for error/warning lines — the one editor decoration VS Code does not paint from published diagnostics.
• Diagnostic navigation bar — error/warning counts labelled "in view" plus next/previous cycling that reveals and scrolls to each marker. Also bound to F8 / Shift+F8 while a preview is focused, scoped by a biofmtPreviewFocused context key so it never shadows the editor's own F8.
• Preview-viewport validation — preview panels now report their viewport, so viewport-aware validation covers what the table is showing. Editor and preview viewports are tracked per source and validated as their union via a new VisibleRangeTracker.

Design notes

• Counts are labelled "in view" because validation is viewport-bounded — we never imply the whole file was scanned, consistent with the project's lazy-parsing design.
• The diagnostics transform (src/shared/diagnostics.ts) and VisibleRangeTracker are kept free of any vscode import, so they unit-test under Mocha.

New modules

• Extension: services/DiagnosticBridge.ts, services/DiagnosticGutter.ts, shared/diagnostics.ts, two gutter SVGs under media/.
• Server: visibleRanges.ts (VisibleRangeTracker).
• Webview: diagnostics/DiagnosticsContext.tsx, components/RowStatus.tsx, components/DiagnosticNav.tsx.

Testing

• Unit (Mocha): diagnostics transform (severity/code/range mapping, worst-severity) and visible-range union/delete/isolation — test/unit/diagnostics.test.ts, test/unit/visible-ranges.test.ts.
• Component (Jest + RTL): DiagnosticsContext grouping/counts/worst-severity, RowStatus glyph/tooltip/click-to-reveal, DiagnosticNav counts and cycling.
• Full suite: 802 unit tests and 60 webview tests pass; lint, all three type-checks, and the build are green; generator drift check clean.