feat(log-viewer): give the inspector's empty state per-source copy

[lukecotter]

Closes #25.

The Inspector rendered one hardcoded emptyText="Select a row to inspect it." regardless of which tab was active, so a user on the timeline was told to select a "row".

detailSections.ts now owns an EMPTY_TEXT map keyed by DetailSource and exports emptyTextFor(source), which LogInspector renders for the active source. Copy names what to click per tab (frame on the timeline / frame in the call tree / row in the analysis grid / SOQL, DML or SOSL row). An undefined source falls back to the old generic string.

One mechanism, not two — the null-selection branch of buildDetailSections still returns [] unchanged; only the placeholder text is source-aware.

Precedence rule

This is the module that decides what "nothing selected" means, so the rule is documented here for #26 to inherit rather than re-decide: an explicit row/frame selection always wins. A range or other ambient scope only applies when selection is null, and must be layered into that branch — never made to override an explicit selection.

Tests

• detailSections.test.ts — the null-selection test loops all four sources; new emptyTextFor describe block covers each source plus the fallback.
• LogInspector.test.ts — steps through all four tabs and asserts the rendered placeholder changes.

Not in scope

DMLView/SOQLView/SOSLView never emit selection: null on deselect, so the database tab can't currently reach its own empty state by deselecting. That's an emission gap in three separate views rather than part of this seam — filed separately.