fix(query-result): correct row classification and stabilize result table UI

[Blankll]

Summary

Fixes two defects in the query result experience:

1. SELECT queries preceded by SQL comments returned no data — only N row(s) affected.
2. Result table Actions column / copy-export buttons were unreliable — the Actions header vanished on wide results and copy/export buttons disappeared entirely when there was nothing to copy.

Root cause & fixes

1. Row classification (src-tauri/src/database/*)

Statement routing used a naive first-keyword prefix check (trim().to_uppercase().starts_with("SELECT")). A SELECT preceded by a -- comment line fell through to the execute path, where driver-level execute() discards rows and only the command-tag count survives (SELECT 2 → "2 row(s) affected").

• New shared module src-tauri/src/database/sql_classification.rs: strips leading comments/whitespace for a fast keyword scan, then falls back to dialect-aware sqlparser parsing for a single Query or RETURNING DML. Inconclusive cases stay conservative on the execute path.
• Wired into the postgres, mysql, and sqlite adapters; their keyword lists are unchanged (postgres additionally accepts TABLE).

2. Result table UI (DataGrid.vue, QueryResultPanel.vue)

• The Actions column header lived outside the rows scroll container, so its sticky never applied and it did not track horizontal scroll — on wide results the Actions column visually disappeared from the right edge. The header viewport now mirrors the rows' scrollLeft, keeping the sticky header pinned at the visible right edge, aligned with the sticky row cells.
• Copy/export buttons were v-if-hidden when there were no rows; they now always render and are disabled (grayed out) instead, matching the tab buttons in the same panel.

Test evidence

• cargo build ✅, cargo test --lib sql_classification — 11 new unit tests (comment prefixes, block comments, unterminated comments, DML ± RETURNING, multi-statement, parenthesized query, VALUES, TABLE) ✅
• cargo test --lib database:: — 242 passed ✅
• npm run build (vue-tsc + vite) ✅
• npm test — 34 suites / 611 tests passed ✅

Notes

• Independent of feat(ssh): full tunnel parity with dockit — chains, profiles, proxies, TOFU #174 (ssh tunnel parity); branched from master, no overlap.
• rustfmt-clean on the touched files only; some pre-existing files on master are not rustfmt-clean and were left untouched.