feat: script export — multi-statement follow-up to streaming Export (#99)

[BorisTyshkevich]

What & why

Follow-up to #87. The editor's Export button only handled a single query — running Export on a multi-statement script silently no-oped/blocked. This adds a dedicated script-export flow:

• exportEntry() dispatches by statement count: one statement keeps Export button: stream full query result to disk as TSV (File System Access API, uncapped, bypasses the grid) #87's single-file exportDirect path; more than one opens exportScriptEntry.
• A directory picker (showDirectoryPicker, requested before any auth/config await — transient activation) is skipped entirely when the script has no row-returning statements.
• exportScript runs statements sequentially in one shared ClickHouse HTTP session (sessionParamsFor), so SET / CREATE TEMPORARY TABLE state carries across statements. Row-returning statements stream uncapped to their own file (NNN-slug.ext, via the new pure scriptExportName helper); non-row statements run for effect and log OK/error with no file.
• The log lives in tab.result.scriptExport — metadata only, never the exported rows, so a multi-million-row script export stays flat in memory. A new results.js branch (buildToolbar + renderScriptExportGrid) renders it live: #/Statement/Type/Status/File/Bytes/Time, ticked every 200ms.
• Cancel aborts the active statement's own abort/query-id (kept separate from both the grid-run and single-export state), issues its own KILL QUERY, marks the rest Skipped, and keeps already-completed files. Stop-on-first-failure, no retry — a partially-written file shouldn't be silently re-attempted.
• ch.exportQuery (from Export button: stream full query result to disk as TSV (File System Access API, uncapped, bypasses the grid) #87) already forwarded params, so session_id rides alongside query_id with no net.js change beyond a regression test.

Two re-entrancy/leak bugs found and fixed during self-review before this PR: exportScriptEntry now flips app.state.exporting before its own directory-picker/auth await (mirroring exportDirect, closing a double-click race); the live-render setInterval now starts inside exportScript's try so a throw can't leak it.

Manually verified end-to-end in a real Chromium browser (real editor, real button clicks, injected fetch/picker seams) — single-statement still uses the save-file picker, a 3-statement script opens the directory picker and produces exactly the expected per-statement file/log output, and Cancel mid-export correctly marks Cancelled/Skipped while keeping the completed file.

Closes #99

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency
• README / CHANGELOG.md ([Unreleased]) updated
• Reconciled affected tracked work (roadmap Roadmap to 1.0.0 #68 updated: Export button: stream full query result to disk as TSV (File System Access API, uncapped, bypasses the grid) #87 marked merged, Export script: run multi-statement SQL with log pane and multiple output files #99 added)

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