fix(web): preserve computed definitions during rule metadata edits

[Maya-Kid]

Computed rules currently appear without their expression, and editing them through the ordinary form sends a definition without conclude_expr, which the API rejects. Render their expression with attribute labels and preserve their definition by editing only name and description. Unknown expressions and values that the ordinary form cannot round-trip use the same protected metadata editor; failed saves retain the draft for retry.

This addresses the existing-rule editing portion of #488. It does not introduce a computed-rule creation editor or decide unit/datatype semantics. Expression operands are displayed defensively, without claiming the current API accepts them on creation.

Validation:

• Six frontend regressions cover expression parsing, depth limits, ambiguous/missing labels, guarded definitions, and the exact metadata-only PATCH payload on failure/retry.
• A real PostgreSQL authenticated API regression reproduces the old 422 response, verifies metadata updates preserve the nested expression and condition groups, and checks invalid-name/viewer rejection.
• Linux workspace: 952 passed, zero failed, one existing ignored public-network test. Final added API assertion rerun passed. Formatting, Clippy with warnings denied, and workspace build passed; frontend 97 tests and production build passed.
• Headless browser checks using mocked API data cover rendering, cancel, failed-save retry, unknown expressions, and Chinese text.
• Combined with the human-review lock fix and rule dependency viewer: Linux workspace 957 passed, zero failed, one existing ignored test; frontend 106 passed, production build and both browser scenarios passed. Combining the two UI PRs requires retaining both imports, translation entries, and helpers in RulesPanel; this union was tested locally.

Refs #488